It's impossible to tell the exact number of people who will stand at the Last Judgment. Only the Lord God knows the exact number of them but we can speculate that there will be tens of billions of them. All of them will be gathered in one place on the new Earth.
This is how this event is described in the Holy Scriptures:
5 ... and Jehovah my God shall come, and all the holy ones with thee.
6 And it shall come to pass in that day, that there shall not be light; the bright ones shall withdraw themselves:
7 but it shall be one day which is known unto Jehovah; not day, and not night; but it shall come to pass, that at evening time there shall be light.
9 And Jehovah shall be King over all the earth: in that day shall Jehovah be one, and his name one.
10 All the land shall be made like the Arabah, from Geba to Rimmon...
1 Behold, I send my messenger, and he shall prepare the way before me: and the Lord, whom ye seek, will suddenly come to his temple; and the messenger of the covenant, whom ye desire, behold, he cometh, saith Jehovah of hosts.
2 But who can abide the day of his coming? and who shall stand when he appeareth? for he is like a refiner's fire, and like fuller's soap:
3 and he will sit as a refiner and purifier of silver,
5 And I will come near to you to judgment; and I will be a swift witness against the sorcerers, and against the adulterers, and against the false swearers, and against those that oppress the hireling in his wages, the widow, and the fatherless, and that turn aside the sojourner from his right, and fear not me, saith Jehovah of hosts.
17 And they shall be mine, saith Jehovah of hosts, even mine own possession, in the day that I make; and I will spare them, as a man spareth his own son that serveth him.
18 Then shall ye return and discern between the righteous and the wicked, between him that serveth God and him that serveth him not.
7 Hold thy peace at the presence of the Lord Jehovah; for the day of Jehovah is at hand: for Jehovah hath prepared a sacrifice, he hath consecrated his guests.
8 And it shall come to pass in the day of Jehovah's sacrifice, that I will punish the princes, and the king's sons, and all such as are clothed with foreign apparel.
9 And in that day I will punish all those that leap over the threshold, that fill their master's house with violence and deceit.
10 And in that day, saith Jehovah, there shall be the noise of a cry from the fish gate, and a wailing from the second quarter, and a great crashing from the hills.
14 The great day of Jehovah is near, it is near and hasteth greatly, even the voice of the day of Jehovah; the mighty man crieth there bitterly.
15 That day is a day of wrath, a day of trouble and distress, a day of wasteness and desolation, a day of darkness and gloominess, a day of clouds and thick darkness,
16 a day of the trumpet and alarm, against the fortified cities, and against the high battlements.
8 Therefore wait ye for me, saith Jehovah, until the day that I rise up to the prey; for my determination is to gather the nations, that I may assemble the kingdoms, to pour upon them mine indignation, even all my fierce anger; for all the earth shall be devoured with the fire of my jealousy.
9 For then will I turn to the peoples of a pure language, that they may all call upon the name of Jehovah, to serve him with one consent.
1 Blow ye the trumpet in Zion, and sound an alarm in my holy mountain; let all the inhabitants of the land tremble: for the day of Jehovah cometh, for it is nigh at hand;
2 a day of darkness and gloominess, a day of clouds and thick darkness, as the dawn spread upon the mountains; a great people and a strong; there hath not been ever the like, neither shall be any more after them, even to the years of many generations.
3 A fire devoureth before them; and behind them a flame burneth: the land is as the garden of Eden before them, and behind them a desolate wilderness; yea, and none hath escaped them.
4 The appearance of them is as the appearance of horses; and as horsemen, so do they run.
5 Like the noise of chariots on the tops of the mountains do they leap, like the noise of a flame of fire that devoureth the stubble, as a strong people set in battle array.
6 At their presence the peoples are in anguish; all faces are waxed pale.
7 They run like mighty men; they climb the wall like men of war; and they march every one on his ways, and they break not their ranks.
8 Neither doth one thrust another; they march every one in his path; and they burst through the weapons, and break not off their course.
9 They leap upon the city; they run upon the wall; they climb up into the houses; they enter in at the windows like a thief.
10 The earth quaketh before them; the heavens tremble; the sun and the moon are darkened, and the stars withdraw their shining.
11 And Jehovah uttereth his voice before his army; for his camp is very great; for he is strong that executeth his word; for the day of Jehovah is great and very terrible; and who can abide it?
31 The sun shall be turned into darkness, and the moon into blood, before the great and terrible day of Jehovah cometh..
2 I will gather all nations, and will bring them down into the valley of Jehoshaphat; and I will execute judgment upon them there for my people and for my heritage Israel, whom they have scattered among the nations: and they have parted my land,
12 Let the nations bestir themselves, and come up to the valley of Jehoshaphat; for there will I sit to judge all the nations round about.
13 Put ye in the sickle; for the harvest is ripe: come, tread ye; for the winepress is full, the vats overflow; for their wickedness is great.
14 Multitudes, multitudes in the valley of decision! for the day of Jehovah is near in the valley of decision.
15 The sun and the moon are darkened, and the stars withdraw their shining.
Thus, the Scripture gives us the name of the place of the Judgment Day - the valley of Jehoshaphat which is called the valley of judgment. Since the Last Judgment belongs to the age to come, it will take place under new time-space related conditions, on the new Earth, the quality and dimensions of which we can't even imagine, according to the new laws of creation. That's why there is no need to worry about the lack of time for the judgment or of room for billions of people. There will be enough room on the new Earth for the saved peoples, too. With God, everything is planned and scheduled. It is not for us to know all the details of the beginning of the age to come and of eternal life. The world will be absolutely different and we ourselves will be different, that's why we can talk in dim and partial terms about those events.
As for the process of judgment of billions of people, it will not take long, in earthly terms. The Lord will not be examining the case of each and every one and then read individual sentences. He will, as He Himself said in the Gospel of Matthew, immediately divide all judged people into two categories. One of them are the righteous who are destined for justification and the other one is the group of the ungodly, doomed to condemnation.
The judgment will be perceived as individual by each judged person. To this end, two books will be opened for each person (obviously, this must not be taken literally but points out to a special way of conveying the information about sins that we cannot yet fathom) - the book of works and the book of intents of the heart. All of the person's works will be compared to the state of their heart when accomplishing them and thus every work shall be judged. Everything will be taking place under totally different conditions and with different possibilities and thus will be relatively fast. According to the overall preponderance of either good or evil deeds, a man is determined into either sheep's side (to the right) or goats' side (to the left). The holy God-pleasers will also participate as judges of certain groups of people. For example, the twelve first Apostles, according to the word of the Lord, will be judging the twelve tribes of Israel.
The saints will also judge the fallen angels (demons).Their judgment will be in full concordance with the will and righteousness of God.
It's hard to say anything about the exact amount of time the Judgment will take (for there will no longer be time in the commonsense of the word), but it will take no longer than 30 minutes, according to earthly time.