You probably wouldn't use it in production for that many people. You can check the Hypersonic project in SourceForge for yourself for more information on its capabilities. The file storage format isn't that great at the moment either.
You are probably better off using MySQL for a robust and scalable DBMS. And MySQL has a good DBMS console to work with if something breaks or if you need to do a lot of administration and backups.
Or use Postgresql. Your choice though.