There is no set limit to the number of plugins you can install on your server.
However, due to the variety of plugins and their different requirements, it’s impossible to give a meaningful number to the question "How many plugins can I use?"
Each individual plugin will use resources (CPU and RAM) on your server to run. The vast majority of plugins use very little of each, especially those with fairly basic features or that make minor changes to gameplay.
Plugins that do more will need more of both. Logging plugins such as LogBlock or terrain editors like WorldEdit will both use a lot of RAM and CPU time as they need to load large amounts of data into memory and perform calculations on it.
To help with performance issues on your server, check our guide on troubleshooting lag.