How many are too many? Well... I should be
around 160 plugins in my project. (And none of those are battle related. As my game is a story-focused game.)
Here's me showcasing around 56 plugins in the first 5 minutes of the game.
Will be releasing the tech demo of the game this month. To make sure the game even runs on people's PC.

(Before continuing with the project as is.)
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There doesn't seem to be any hard limit on using plugins. You can use as many as you'd like.
Don't expect all plugins to just work together though.
The thing is with MV, a lot of the plugin have matured, repeatedly updated, and have been patched to work with one another.
And there's an alternative to most plugins out there. So if one doesn't work, you can use an alternative that does work.
If two plugins aren't compatible, then perhaps you can turn off (or clear out) a feature in one of them, that's causing incompatibility. (As a plugin can have several features.)
That's what I've been doing. With every game, I'm adding in more features though plugins. And so increasing the plugin count.
The plugins in my collection are around 6000. I've only managed to plug 160 of those in my project.
And now I have to go through the whole list of 160 plugins, in every scene, to figure out "Okay, what else can I use here?" XD