The most outfits I've had in the formalwear category is 2. I usually have a normal formal outfit and then add a wedding dress to my female sims. My male sims only get 1. I have a couple restaurants that require formalwear.
I generally only give my sims 1 outfit per category because I've found my game runs smoother when rotating without all the extras. When I get tired of an outfit, I'll just change it out. With mannequins in retail shops, there are 5 outfits per category because I like to change the mannequins' outfits per season or when one sells.