How many extensions can I install?
Joomla has no limit in the amount of extensions (plugins, components, modules and templates) you can install.
Your website will work fine as long as all the extensions are compatible with Joomla, each other and the server environment. However, never install unfamiliar extensions in live websites.
Try in a separate installation
The best way to discover if an extension really fits your needs is by trying it yourself. It's good to start exploring an extension in a separate Joomla installation. Or, even better, in a local copy of your live website.
Using our Vagrant box you can install new Joomla sites locally in seconds.
Uninstall what you don't need
Make sure to check all installed extensions before moving the site into production. Maybe the requirements of the client changed and you had to use another extension but forgot to uninstall the previous one.
Do a full backup before doing anything so you can always restore any error. Oh, and don't forget to backup, the backup!
Even the best Joomla extension can break and often in the worst possible moment. Make sure every installed extension is actively supported. So you won't be on your own when you encounter a problem.