I’m building a big Maze caves system. I was wondering how much unreal can handle because I’m not half done and I have around 200 megascanes meshes
Much of your performance will be determined by how much load you throw at the underlying hardware vs what you throw at Unreal (it really is more taking what you throw at it and throwing it at the hardware for you.).
If you are using instanced-meshes then the reply is ‘quite a few’, but again, it depends on if those 200 meshes each have 200 or 200,000 vertices, the materials, etc.
Not to sure what your root worry is as although 200 megascan meshes sounds like a lot Unreal 4 can handle a lot more and with UE5 on the horizon promises to improve even more with out concern for overloading on environmental detail.
Of general interest Unreal 4 is a closed edit environment so the fidelity of assets imported should be of a quality that exceeds the desired output as in the ideal that you can make to much do less than trying to make not enough do more and Megascans do a very good job of importing into Unreal 4 the necessary requirements by including LOD’s as well importing materials as an instance of the first material imported.
This is to say that if you give UE4 what is required it will handle a lot of the performance requirements for you with little or no loss to fidelity of asset. UE5 promises to do a better job.
Granted this still leaves the question “:where does the deep end begin?” and sad to say it’s really a limit you have to figure out on your own via discovery as there is very little documentation as to performance tuning but as a tip I found through my own experience that it’s best to worry about the overall performance once the environment become closer to being completed and you can run performance profiles and fix the things than needs to be fixed rather than worrying over the idealism that 200 meshes will tank your performance. 
Something I did back in a day
The main question is how much memory the user’s machine has. If you split assets appropriately, and use level streaming (either manual, or through world composition,) you should be able to make “very large” levels.
Also, if you’re doing caves/tunnels, it may make sense to place lots of pre-calculated visibility zones in the level where the player will run around, to reduce the amount of geometry it will draw in any one instance.