Then I look, and there was the Lamb, standing on mount Zion! And with him were one hundred forty-four thousand who had his name and his Father's name written on their foreheads.
Revelation 14:1
ESV - 1 Then I looked, and behold, on Mount Zion stood the Lamb, and with him 144,000 who had his name and his Father's name written on their foreheads.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
The 144,000 are there from the 12 tribes of Israel from the old covenant. It's a symbolic number but it is complete. But as he looks around at the multitude from every tribe and nation that he cannot count these are from the new covenant this people are us the Gentiles. But the fact that he cannot count them is that more and more christians are being saved to this very day. Blessing.