Mpd (used by Volumio for the database & playback) does not have a limit on number of files as far as I am aware. However, there are a number of posts on these forums about such issues. These can be caused by files that mpd is “unhappy” with & the scan finishes prematurely. Also, there seems to be a problem with very large storage (>2.2 TB) without appropriate formatting. I have also read elsewhere that the size of the sd card (Volumio OS) can affect successful scanning by mpd … size of database is often cited, but I think this unlikely when we are talking about GB of storage space.
If i were experiencing such problems, my approach would be to try & introduce my file collection by a section at a time … this would throw up bad files & also probe space constrictions. Please report back and let us hear how you get on.