How many add-ons are too many to be running at the same time. Does running many add-ons affect the site responsiveness? I was growing a bit concerned as I would need to use a lot of them to get my site to where I want it to be. Have you encountered any negative effects?
You can run quite a few add-ons before you start running into scalability issues (assuming all add-ons do zero queries). Most notably, the event listener system fires a call to check if it should run for every possible time *anything* is run. For example, if you have 50 add-ons that have any sort of template hooks in them and a page that has 40 template hooks that *could* run, you end up firing 2,000 static methods simply to check what should run (even if nothing actually does). I've made a few minor changes to the event listener system so that it's only firing events for things it actually needs and it made my pages noticeably faster (went from firing ~2,500 extra static methods per page view to around 5-10).
But even without the change, it certainly doesn't make the site so slow it's unusable. The bigger thing to watch out for is just poorly designed addons that make a lot of queries and don't utilize caching.