It's not really about how many plugins you can install on wordpress. It's about three factors:
- quality of the plugin (is it well made? is it maintained etc)
- the scope of the plugin (do you really need it? does it add value to your website? etc)
- wordpress hosting. Here the discussion can be very long and complex but I'll keep it short. If you're hosting your website on a powerful VPS, you can add more plugins because you know the server will be able to cope with them. IF you're on a shared hosting, don't go nuts with the number of plugins. Look for what you really need and try to install plugins that are light when it comes to consuming hardware resources (read the reviews for each plugin)