Upvote:1
The differences between Ryokan and Minshuku have become blurred over time. While back in time, they were worlds apart, one serving members of the court while the other served the lesser ranked members of the entourage, when the court traveled between castles. Today they are not as distinctively different. And using "ryokan" in a hotel name is cheesy marketing ploy to attract more uninformed tourists.
But to your points, not all minshukus provide private rooms, some place several travelers in a shared room. Some serve food, some don't. You basically have to research each one to see if it offers what you want at a price you can afford.
The advantage of a real minishuku over a hostel is atmosphere, you would be sleeping on a futon on the tatami mat floor, enjoying a hot water tub for soaking, more traditional Japanese dinner. A hostel would be western style beds, basic bathrooms, head out for fast food at dinner time.
As far as number of guests, that would be determined by the facility's capacity, not type as you can find both large and small versions of each.