When you specify the number of databases for an account, that is the amount of databases a user can create for each MySQL and PostgreSQL. This means assigning a limit of 3 SQL databases allows the creation of 3 MySQL databases and 3 PostgreSQL databases (rather than a combined total of 3 SQL databases).
We are currently considering having separate limits for MySQL and PostgreSQL since this is counter-intuitive to some. Internal Case 389.
EDIT: This was revised in the early builds of 11.30 per case 389