There is no cut and dry answer unfortunately. Help:Category Names states that:
"Ideally, bottom-level categories should be much smaller, with no more than a few thousand people in them. This isn't always possible, e.g. with towns and cities, but we should aim to create narrow categories when we can."
There was a comment previously about Wikipedia, but it should be noted that this is actually much more restrictive than Wikipedia in most cases. Wikipedia currently has:
- 700+ Categories with more than 20,000 pages
- 750+ Categories with 10,001–20,000 pages
- 1,400+ Categories with 5,001–10,000 pages
- 3,330+ Categories with 2,001–5,000 pages
While I don't have any exact numbers for WikiTree, I do know of a few large categories (based on the last dump) such as:
- Category:Unsourced Profiles - 1,020,064 profiles
- Category:Notables - 30,476 profiles
But as you can imagine, this is not typical. I am confident that our average numbers are much lower than this, and there are very few categories that reach over 5,000 profiles.