What is the cost of an account on disk when it is created in MySql?
Practically nothing (a few K to create the rows in the Grant tables).
what is the max number of accounts you can create?
Practically unlimited (the Grant tables can't exceed the maximum number of rows in a table, for which there is no upper limit in modern InnoDB tables, but for obscenely large numbers of rows you may have performance problems. The chance of you having that many users / GRANTs is so small as to be a non-factor in capacity planning).