Custom Web Publishing, unlike Instant Web Publishing, does not use persistent database sessions. This means that a user is only a user for as long as they're making a call to the database server. So even with 500 people on the CWP site at the same time, 100 of them would have to perform an action that interacts with the database server at the same exact time in order to reach that limit.
TSBear wrote... "So even with 500 people on the CWP site at the same time, 100 of them would have to perform an action that interacts with the database server at the same exact time in order to reach that limit."...
We are expecting more users than 500 and quite possibly in the 10's of thousands and can forsee this limitation being exceeded, especially since it is intended to be fairly interactive.
No other advice other than choosing an alternative database?
There's no *easy* way to expand past FileMaker Server's limitations. You could use multiple servers, then use SyncDek to sync them together, but that won't be absolutely realtime.
Honestly, if you are going to have as much traffic as you expect, you need to look at using MySQL or a similar database for your web back-end. Note you can still integrate with FileMaker by using the ESS (external data source) feature.