I don't think there is really a maximum cap for plugins, but you should try to minimize your plugin use as much as possible. This is because:
- More plugins means more maintenance.
- Plugins don't always get updated in a timely manner.
- Plugins get abandoned.
- Plugins are rarely efficient.
- Plugins are rarely secure.
- Plugins may conflict with eachother.
- Plugin updates may break your theme or your Wordpress install.
- Plugins aren't always easy to rollback.
I'm not saying don't use plugins, but I am saying be careful.