Each package has a healthy plugin range: Starter 12, Standard 20, Pro 30. Ultra and above (and custom configs over 8 GB) are unlimited.
These limits aren't arbitrary: every plugin consumes RAM and tick time. The limit protects your server's 20 TPS.
Heavy plugins (dynmap, big economy systems) can weigh as much as several ordinary ones. If TPS drops, check the Statistics tab to see when it started.
Need more plugins? Upgrade your package or raise RAM in a custom configuration — above 8 GB the limit disappears entirely.