Too many activated fonts - identify those in use and remove/archive others
MacOS comes with fonts installed in two locations:
/System/Library/Fonts

/Library/Fonts

The fonts in the /System location cannot be disabled**, and are required for the 'safe' rendering of all text that the interface needs.
The fonts in /Library/Fonts can all be disabled in Font Book. This location is also where some applications install fonts that they use. (Though well-behaved apps will store fonts that they need inside their bundle, so they are always available, but not shown in the font menu.)
It is possible to open a document that requires some font which is not currently enabled. Some apps may give you a warning of that fact.
** Catalina introduced a 'Supplementary' folder to the  /System fonts folder, some of whose fonts can be disabled.
There are third-party font managers which offer greater degree of control and 'auto-activation', where fonts can be enabled when requested.