Check this thread with answers from /u/amp1212 and /u/tiako
https://www.reddit.com/r/AskHistorians/comments/emx7my/how_many_roman_legionnaires_survived_to_retirement/
You may also want to check the paper Tiako references, here:
http://www.princeton.edu/~pswpc/pdfs/scheidel/110509.pdf