It depends on the type and topic of the wiki. I am not sure where you got the idea of 50 being a minimum. Some wikis have only 100 or so and others have hundreds of thousands. It all depends on the amount of content there is to document.
Even if the average page could was a good measure, that data isn't available to users.
The 50 is not a minimum, but it is a good place to start when you start a wiki. If you have 50 (substantial) pages, it means that there is quite a fair bit of content on the wiki, and it's often taken as first milestone (once again, not a minimum). There are wikis with thousands of pages, and there has even been wikis with over a million pages. However, you shouldn't build your wiki just based on number of pages - there are wikis with thousands of pages that were created with a bot or that were created as empty - don't do that.
You can have as many pages as you need. If you're a wiki on a small indie game that just came out, 50 (or less) might just be your maximum. If you cover a franchise that's been around for half a century, 50-100k not unusual.