Revelation 20:7 And when the thousand years are expired, Satan shall be loosed out of his prison,
8 And shall go out to deceive the nations which are in the four quarters of the earth, Gog and Magog, to gather them together to battle: the number of whom is as the sand of the sea.
9 And they went up on the breadth of the earth, and compassed the camp of the saints about, and the beloved city: and fire came down from God out of heaven, and devoured them.
10 And the devil that deceived them was cast into the lake of fire and brimstone, where the beast and the false prophet are, and shall be tormented day and night for ever and ever.
Now the 1000 year reign is coming to an end when Satan is released to deceive as many as he can. You ask how can the people now be deceived that have lived under the reign of Christ and the saints for 1000 years and how many will be deceived? When Joseph’s family came to Egypt they were 70 people, in 400 years that tuned in to over two million people, now more than 70 people go in to the millennium and have children for 1000 years years so there could be billions of people on the earth now. Also you have to realize that man still has a sinful nature and even though Christ reigns Satan’s temptation can still turn people so the army he gathers this time is more than the sands of the sea. Satan went world wide gathering the people to fight with him and they came and encircled the city of God, Jerusalem, but as the first battle it was no contest, God breathed fired down from Heaven and destroyed them all and the devil was cast into the lake of fire where the beast and the false prophet has been waiting for him 1000 years.They will be tormented day and night forever in the lake of fire never to deceive man again.