Since Wildfire 2.4.1 the max number of database connections established to the embedded DB is 25. That means that you may have up to 25 SQL statements running at the same time.
Database access is usually required when logging in unless you are using anonymous users. So if you have many hundreds of users logging in at the same time (not highly likely unless the server was suddenly restarted) then you may experience some delays. For normal usage (e.g. message exchange) db operations are not required so you should be fine hosting many thousands of users.
Having said that, if you are planning to have more than 500 concurrent users I would recommend moving to an external database. Databases like mysql, ms-sql, etc. include tools that a real production environment might need like warm or cold back ups, mirroring, failover, etc…