One of my favorite New Testament commentaries were done by William Barclay. He noted that in Hebrew numbers could use other meanings, not just a count. For example, Twelve is the number of completeness (All twelve tribes). Notice that twelve was not just doubled, it was squared to 144. Even that was not enough for the author--he added a thousands to 144 to make it 144,000. What he is saying s that the number who will be with Jesus is great--complete and overflowing.
Hebrew letters themselves also had other meanings, not just numbers.