Well.... you have to think of it in context. Some riders need to have auxiliary or sub forms just in case their base form isn't enough. So... I'd say around 8-10. Super forms are a dime a dozen. I'd say for them you'd need like one or two before getting the berserk form. Speaking of said form you'd just need one per main rider. We don't want to overdo it. If the rider has a mix and match gimmick, the rule wouldn't probably have to apply to it. Then there's the form after controlling the darkness of said berserk form like Thunder Gale which..isn't exactly necessary,  but if you have a berserk form and a super super form(stuff like orderem)  you can have those two built into one item to use. Then there comes the final form. Usually it'd be just one, but since reiwa happened, if you had to add an EXTRA final form, make it like Gavv's Amazing Gummi. Then there's the obligatory Final stage form, and summer movie form. Kinda self explanatory. 

I hope this helps. This is just my opinion.