Hello,
I guess it depends on the machine(s) which serves your KC Installation(s), theoretically all permutations on alphanumeric pattern with a length of 255 characters (for the name) but the id is a UUID based on 36 character length.
AND the size of one realm depends - practically speaking - on the amount of groups, users, clients and so on and during runtime based on the logged - in - users (session management).
Your question is not so easily answered; it just depends on your hardware and your configuration.
With best regards
Bjoern