I don't think there are any absolutes about this. So much depends on where you live, what your roles are, what your funds are, what your priorities are!
For some people, small is 10 items. Or 10 items per season, excluding core basics like underwear. For others, small would be around 50. Or 100. Still others might consider their wardrobe small at 150 pieces -- and if they live in a place with highly varied seasons and they have to fulfill a number of roles in their lives that don't really allow for a lot of wardrobe crossover, that might be true.
I don't even know the total number of pieces in my closet. I consider myself to have a "medium" sized wardrobe because I have more than I strictly speaking *need* to get by...but less than would stuff my small closet!
More seriously, I worked out recently that I had about 40 pieces excluding workout gear, layering Ts and the like and footwear for my last 4 month period. That felt about right for my needs.
(I don't mean I had 40 pieces exclusive to that season -- I do use items from one season in the other seasons...but not all items cross the seasons).
And I probably have more for my fall-winter closet because that is my longer and dominant season.
So if you add them up, maybe 100 or so? I consider that medium sized.