I picked 10-15m. Earlier Nintendo exclusive Monster Hunter games sold like 4.3m tops, but they were on 3DS, and Switch sales have already surpassed 3DS. So far other handheld focused Nintendo series like Animal Crossing and Pokemon have sold more on Switch than on 3DS, and Monster Hunter should as well, plus Rise is getting a year late PC port which will boost it's sales compared to the fully Nintendo exclusive 3DS MH games. Also Monster Hunter Rise will be boosted by the fact that the popularity of the Monster Hunter series has increased thanks to MH World's breakthrough success (16.8m sales so far), which will in turn have a carryover effect to the success of MH Rise.
I don't think that Rise can top World in sales since World is on 3 platforms as opposed to 2 for Rise, but I do think that it will get pretty close to 15m on Switch and PC combined by the end of Switch's lifecycle. If Nintendo only paid for timed console exclusivity and Rise can be ported to Xbox and PS later on in say two years, then I definitely think it will outsell World.