The number cannot be taken literally. Even the literalism formerly taught by the Jehovah’s Witnesses cult (prior to its own growth spurt) has been revised. If not a literal head count, the number must be symbolic, and likely meant to point to the completion of God’s work of salvation among all the nations.
The symbolic of the number derives from the twelve tribes of Israel, multiplied by itself (the church), multiplied by one thousand (sometimes taken as a reference millennial believers) – equaling the sum of all the earth’s redeemed, those chosen and affirmed as sons and daughters of God.
REVELAITON 14:3 – 3 And they sung as it were a new song before the throne, and before the four beasts, and the elders: and no man could learn that song but the hundred and forty and four thousand, which were redeemed from the earth.