As an eventer, I will say a lot depends (which is the case for most things horses). Some horses are very sure footed and balanced and can comfortably go in less than perfect going with little to no studding. My own horse is very cat like, and always needed very little in studs. I was often a step or two down than a lot of people. Some horses absolutely can not tolerate losing their footing, and need a lot more help. It’s a bit of a trial and error, guessing game until you figure your horse out. I always recommend the “less is more” approach when you’re starting out. Even just studs on the back or just on the outside (most Europeans, at least in eventing, don’t stud on the inside). Sometimes all you need is a little something pointy on the outside of the hinds.
I inherited my old boss’ stud box when he retired, and while I have a HUGE assortment, I can say that, even when I was eventing seriously, I used mostly the same three or four types, all relatively small, all the time. Once and awhile, I’d go a bit bigger for mud, but I have HUGE mud studs that I’ve never taken out of the kit. My feelings on those were always “if I need that much stud, why the hell am I riding?!”