The JW book "What Does the Bible Really Teach" asserts that (the blood of the) "covenant, or contract, makes it possible for 144,000 faithful Christians to go to heaven (p207). and "These 144,000 Christians, including Jesus’ faithful apostles, are raised to life in heaven" (p74). This question seeks the Biblical basis for this (apparent) "144,000" limitation on how many Christians can go to heaven, particularly given Rev 7:9,
"After this (the sealing of the 144,000), I looked, and behold, a great multitude which no one could count, from-out-of every nation and all tribes and peoples and tongues, standing before the throne and before the Lamb, having-been-previously-clothed in white robes, and palm branches in their hands. (Rev 9:7)
when the subsequent context suggests this event occurs in heaven:
...all the angels had-been-standing around the throne and the elders and the four living creatures; and they fell before the throne upon the faces of them, and they worshiped the God. (Rev 7:11)
... these (the ones in white robes) are the [ones] coming from-out-of the great tribulation, and they have washed the robes of them and made them white in the blood of the Lamb. Because-of this, they are before the throne of the God; and they serve Him day and night in the temple of Him... (Rev 7:14-15)
Apparently, an uncountable number are observed holding palm branches... in white, washed-in-the blood robes... from out of the great tribulation... standing before the throne of Jehovah (in heaven)... serving Him night and day in His temple.
This report seems to conflict with only 144,000 believers going to heaven. In answering this question, please do not address why 144,000 is considered literal, or that "144,000" are sealed on their foreheads during the 6th seal (Rev 7:4), or a similar "144,000" are subsequently observed in heaven during the 7th seal (Rev 14:1). These have been addressed elsewhere on this site. This question seeks the Biblical basis for the specific (~144,000) limitation, when Rev 5:7-15 suggests the number is uncountable. An acceptable answer needs to harmonize this (apparent) conflict, without introducing non-scriptural information, ignoring anything relevant or contradicting other scriptures.