As João noted really it's not the number of clients that's connected that is important but rather what those clients are doing. You can have tons of clients passively connected to the server without issue as long as users are following best practices and not loading huge timeframes of raw data (e.g. PurePaths, raw web requests, etc...) and don't leave auto refresh on. It is this pulling PurePaths from the server that can put a lot of load on the server.
Even charts which load from the performance warehouse database do not put too much load on the server itself.
Depending on your server specs and the number of purepaths in a profile, even a single user requesting a month's worth of raw data repeatedly can impact a server significantly and I have seen this happen. In these cases you can often see the culprit within the analysis logs on the server.