Wife and I were there this past early Oct. Basically had the place to ourselves. Even in the car campground there were not more then 2 campers (also, some of the car campgrounds were closed for the season). I'd imagine that would be pretty much true in early May given that no permits are needed. If the crowds warranted permits they'd issue them. Call the ranger station in Watersmeet. They are very helpful people. Also, I've been in the BWCA in May and except for the fishing opener weekend it is real quiet.
Bring some cold weather clothing and some warmer weather clothing. It might be 77.12 degrees or it could snow.
Pull-Up Strap Handle Kit
First Need Purifier