I am evaluating SharePoint Foundation 2010 (free edition) for a prospect with 50 users. Does Foundation 2010 limit the number of users somehow? Thank you!
No it doesn't; SharePoint Foundation is Free to use. Only the SharePoint Server (Standard & Enterprise) uses the Client Access License (CAL) model which is limited by CAL licenses per user.
In addition, this CAL model does not technically limit your usage of SP. But, for legal purposes you must ensure you have the right amount of CAL for your scenario.