Who knows, the 144000 may be for the Old Testament people, all those who followed God before the New Covenant, those who had to offer sacrifices for their sins, eg. Moses, Enoch, Abraham, Issac, Ezekiel, David, Daniel, etc, and because of the New Covenant, because Jesus died on the cross, we may even see some OT people like Cyrus in heaven, 144000 may be a selected number of OT people. Or it could just be another symbolic number 12 x 12000. At least God knows, and that's the main thing, not everything has been revealed to us yet, and we do not know how many will get to heaven, or how many will live the Millennium, but I believe the Millennium is a reward for all the Martyrs for Christ, all the Christians who got persecuted, tortured, beaten, killed in India, in Somalia, in Myanmar, in Indonesia for spreading the Gospel and praising God, during the rise of the Soviet Union, during the time of the Atheist rule of Russia alone, 10s of millions were beaten, tortured or killed, so there is definitely way more than 144,000 in the persecuted Church, and probably way more than 144,000 beheadings. There will certainly be a lot more in Heaven than 144,000 though, the size of Heaven could fit in billions. So in saying that the number 144,000 would most likely be symbolic, just like when Jesus said to forgive your neighbour not once but 70 x 7 times, just like a thousand years is 1 day to God, just like the 1st couple of chapters in Ezekiel, as we know the Bible is not 100% literal, it's a very supernatural book, and yes while somethings did happen in the natural, a lot happened in the supernatural too, and there is stuff that is still to happen.
But you know why do Christians carry on so much about the end time and book of Revelation? We have only been in the end time for around 2000 years and God's creation is still pretty young, if you were to go of the Hebrew Calendar which is currently year no. 5777 and divide it by 25 years, that is going of an average of 25 years in between each generation, the number of generations that have lived this earth would be under 250 generations.