I haven’t had any trouble when I reached over 13,000 parts in my place. Though I often tend to space out my structures so that there isn’t a congestion of high brick count per structure. Additionally, I tend to just have the essential part to any structure, and not go overboard with aesthetics such as vines climbing, individual bricks in a wall, ceiling panels, etc.
Before anyone else comments, I want to jump on this to explain that due to the amount of variables in how Parts, Meshparts, Unions, etc all work together, it’s impossible to know any hard number estimate. Variables such as instancing, complexity, transparency, saturation, and much more can all influence how many of anything would be “too much.”
Which brings us to the only actual solution that has the best chance to give you answers: test your game live, in a full server.
Like I can say that usually any game that uses more than 700mb of memory tends to have issues running on mobile and low-end PCs, but there are some games where 500mb is too much and others that can reach 800mb and run “well enough.” So it’s impossible to know for sure without stress testing.