The 144,000 is only ever mentioned in Revelation - and then twice only. It is not mentioned anywhere else in any other book that comprises the Bible. Revelation was authored by a man experiencing some of the most mind-boggling visions ever recorded, replete with vivid symbolism and imagery. And they base their doctrine of a literal number 144,000 on that?!
Sheesh, I should have seen the lunacy in that doctrine alone before age 10.