Some people take this number as a literal, exact headcount - a specific group with a specific ethnic and tribal makeup, sealed for special protection. Others see the number itself (12 x 12 x 1000) as symbolic of completeness - the whole people of God, from every tribe, made complete.
Revelation 7 itself seems to nudge us toward the second reading: John hears the number 144,000 announced from the twelve tribes - but when he actually looks, what he sees instead is “a great multitude that no one could count, from every nation, tribe, people and language.” That pattern - hearing one thing, seeing another that reinterprets it - happens elsewhere in Revelation too (the Lion who turns out to be a Lamb, in chapter 5). It reads less like two different groups and more like the same group described two ways: an army counted for battle, and a worshiping multitude too vast to count.