They believe that only 144,000 can enter heaven
The Revelation of Jesus Christ given to John
And I heard the number of those who were sealed, one hundred and forty four thousand sealed from every tribe of the sons of Israel. (Revelation 7:4)
This 144,000 are on EARTH. This 144,000 are descendants of Israel, 12,000 from each tribe. They will receive the 'seal of the living God' on their foreheads, which will protect them from the coming judgements from God (see Rev 9:4). This has nothing to do with how many 'can' enter heaven.
After these things John then sees
a great multitude, which no one could count, from every nation and all tribes and peoples and tongues, standing before the throne and before the Lamb, (Rev 7:9)
This is a different group from the 144,000. This group is in HEAVEN. This group is without number because it is too large to count. This group is all true believers in Christ. John sees this group in heaven after the sixth seal has been opened. This group has just been raptured from the earth before the day of the Lord.
So we see that in no way is entry to heaven limited too 144,000.
