Is it better to have one large high poly mesh or lots of small lower poly ones?

So I’ve got a large revit building file i’m importing into unreal, is it best to try and import for example the walls as all seperate meshs, or collapse them down in 3ds max to one larger mesh? I’m using dynamic lighting so not worrying about UV and light maps. Previously I’ve been combining them as its easier to apply the materials in one go, but this does come with some limitations. In terms of performance is one higher poly mesh better than two small lower poly meshes?

I was also trying to use mesh distance fields which really did not work with my scene, and I think one of the reasons may have been the combined meshs being too large (visualising them produced bizarre patterns, nothing like the demonstrations and tutorials), so I though maybe having each wall as a separate mesh would be a better option?

Is there a limit on the number of meshs you can have in a scene anyway?