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.