I do not think that quantity matters - but please see also the release notes on an earlier version:
There is a fixed set of genres for ID3V1 tags represented by one-byte-numbers.
There is no such restriction for ID3V2 tags so you are free to do as @LyricsLover described.
These user-defined genres for which there is no one-byte-number will be displayed as "Other" in ID3V1 tags.