How Many plugins can you safely run on one MU site?


I am looking at all the great plugins here on WPMU Dev, and see the value of having quite a number of them on my next MultiSite project. The main plugin I plan on using is ProSites, and there are several others that I would like to add in as well (from WPMU) and then a handfull that are from other sources. Question is, how many plugins is too many plugins? I'm not talking dozens, but is 15 plugins too many? Is there a line, and what are the consequences?

Thank you,