This is a known major issue for a few years now:
Download and install the FLS checker (see top of forum). It tells you how many unique plugins you have left to instantiate on your system in a nutshell. If you use many of the same plugin you can use as many as you like. Some plugin vendors use more of the open system static linking slots that others
Steinberg are carefully moving to help resolve the issue but it also lies at the door of the plugin manufacturers and moreso Microsoft