how many people can a mysql database handle?

i think this question would be better posed on a mysql/php forum, but to be honest, i'd be more concerned about the web servers capabilities than the database opening multiple recordsets simultaneously. in other words, i doubt you should have any trouble.

and as always for optimal database performance it's always good knowing how to get the most out of your database (ie, using indexes on your tables, stored procedures, well formed sql joins, etc.)

MySQL can pretty much handle as many users as you want, though each asynchronous connection can have a significant bandwidth hit. If you are truly planning on having a lot of people then perhaps you should check that query your MySQL installation has query caching enabled, as this can making coping with many simulataneous users much easier.