I've worked on 1.6 million square foot projects and have never seen a file with more than maybe 10 worksets. Mostly for links. IMO if you are asking how many is too many, then you probably have too many. I'm not sure what you would need a ton of worksets for but maybe there are some workflows out there that would justify it.

The "problem" with Worksets is that success is 100% dependent on users putting things on the right workset. The more worksets you have, the higher the likelihood of something being put on the wrong one.
And, the ONLY proper use of worksets is performance management. Every time I have seen (way) too many worksets it's because worksets are being used for visibility control. You are much better off using filters and view types and view templates to manage visibility and all other aspects of variable graphics, which will then drastically reduce your workset count to the point where no one is asking "do we maybe have too many?"

I wrote a blog post called How Many Worksets do I Need some years back. I took some liberty with comedian Jeff Foxworthy's routine of "You may be a r3dn3ck if...".

To add to the other wisdom offered already, each workset should have a clear reason for being, that any person on the team should be able to understand or even infer. I think of Revit's worksharing as a public library. We could put any book on any shelf but that would make it harder to find a specific book because it could be anywhere. In Revit I think of the bookshelf as a way to associate parts of the building or design components with each other logically. This means everything in a model is a book and a workset is a bookshelf.

If I have a building that has two wings, east and west then it can be helpful to assign east and west building elements to separate worksets called East and West (or similar). Then when I open a project I can decide which wing I want to focus on and then close the other wing's workset. That's a global off switch for everything (model elements) associated with that wing. It can substantially improve the day to day experience of working on a large project.

We learned early on that assigning linked files (RVT and DWG) to specific worksets gave us a way to decide which linked files to load before Revit actually does load them (when we use the Specify option in the File Open dialog). This alone can significantly reduce how long a file takes to open, especially if it isn't necessary to see the elements in the linked file(s) to do what I need to do now.

I've seen more than a few projects with quite a few worksets, enough that you have to scroll or expand the dialog. How many there are is less important than each one's presence can be justified, to the benefit of the project and its team.

I use codes from our (Swedens) current building classification system to name worksets. Then as long as you're familiar with the classification system you know what the workset is for. Makes it easier to handle long lists of worksets, should you find a need for them.

What he said.
When we first started with Revit (wow, that was a long time ago) we set up; not exactly a lot, but a dozen or so Worksets.
Wasn't long before we found that they caused more problems than they solved.
The two big problems are that, as Gordon says, its completely up to the user to remember to change their Workset before modeling everything. And that's even worse than the bad (cad?) old days when people would forget to set their Layer. Since there's no visual clue (i.e. color) you're probably not aware that you're on the wrong Workset. And, since there tend to be less Worksets than Layers, it's not at the top of your mind to remember it all the time.
The other big problem is that the default Workset is whatever it is when the last person Synced. So if you come in tomorrow morning and make a new Local Copy (like you should be doing), your Workset may not be they same as it was when you Synched last night.
And finally (OK, 3 problems) having something on the wrong workset is more problematic than having "too few" worksets.

To answer the question "Do I have too many Worksets?", you have to first ask yourself "What problem am I trying to solve by using Worksets?"
If its Visibility, then I would again agree with Gordon that Worksets are the wrong tool. Kind of like using a wrench as a hammer. Sure, it'll work, but not efficiently
If the problem you're trying to solve id performance, then Worksets are the correct tool and you need to evaluate what can be shut off without getting in the way and what would gain you perfomance.
If you're just using Worksets to keep yourself organized, then it might be your process that needs tweaking.

Workset Display shows everything by the workset it is on. Makes it pretty easy to modify it if needed.

So that's what that is for. Good to know.

If only it had a scheme 'key' device like rooms & areas...

Yes, that's great to use.
But it's a "go get it" kind of thing.
If you're going to remember to turn on Workset Display to see what's on the wrong worksets, you've probably already remembered to set the correct workset.
I was talking about live feedback where you would see that something's on the wrong workset the instant you drew it.
Yes, there's "Grey Inactive Worskets", but I don't know of anyone who uses that very often.