AFAIK, there is no such limitation. Indeed, I am able to add as many users as I want:
girish@benton:~/tmp$ ./mc admin user add minio newuser newuser123
Added user `newuser` successfully.
girish@benton:~/tmp$ ./mc admin user add minio newuser1 newuser123
Added user `newuser1` successfully.
girish@benton:~/tmp$ ./mc admin user add minio newuser2 newuser123
Added user `newuser2` successfully.
girish@benton:~/tmp$ ./mc admin user add minio newuser3 newuser123
Added user `newuser3` successfully.
girish@benton:~/tmp$ ./mc admin user add minio newuser4 newuser123
Added user `newuser4` successfully.
girish@benton:~/tmp$ ./mc admin user add minio newuser5 newuser123
Added user `newuser5` successfully.
girish@benton:~/tmp$ ./mc admin user add minio newuser6 newuser123
Added user `newuser6` successfully.
girish@benton:~/tmp$ ./mc admin user add minio newuser7 newuser123
Added user `newuser7` successfully.
Then:
girish@benton:~/tmp$ ./mc admin user list minio
enabled newuser1
enabled newuser2
enabled newuser3
enabled newuser4
enabled newuser5
enabled newuser6
enabled newuser7
enabled newuser