Honestly when it comes to add-ons its usually about having too many themes if you ask me lol. Everytime you install, uninstall or upgrade and it reparses templates it has to go through every single theme I believe. Takes longer with more themes.
Generally there is no number. The right addons I could have 100 and barely notice any difference with 15. An Addon can be a simple as a template mod and you can end up with a few of those so that you don't have to manually apply a common tweak to several themes. There is always going to be additional checks because of each addon but we are probably talking under a millisecond for each one?
It's when you get into having several heavy addons that do database reads and writes and they are all effecting one page. Thats when the performance gets questionable.
It depends on the add ons installed. If the add ons are coded properly then no matter how many you have installed you have nothing to worry about. But if they are poorly coded, then even one is one too many imo.