I actually think difficulty amount limitation can be a good thing, but not as a total number limit.
- If mapping styles are extremely similar for the same difficulty icon (i.e. https://osu.ppy.sh/s/320118 and https://osu.ppy.sh/s/596704), then the number of diffs of that icon should be limited to one; however, if styles for the same difficulty icon do noticeably differentiate (i.e. https://osu.ppy.sh/s/115193 and https://osu.ppy.sh/b/928371), then multiple diffs of the same icon should be allowed.
- Top diff is exempt since it can be considered an Ultra, or because it's the hardest difficulty by star rating.
- Exceptions can also be made if the issue is spread balance.