Great question, because it's directly related to how many clothes we need, isn't it?
When I put my seasonal work wardrobes together, I plan on a two-week rotation for tops and dresses. That is, I plan on having enough tops and dresses to not have to repeat any for at least two weeks. For me, this means I need a minimum of eight tops and two dresses per season.
That being said, I like to challenge myself to seldom repeat exact outfits. I enjoy mixing--combining my shirts with different bottoms and/or toppers, or switching out a necklace for a scarf, changing footwear, etc.
And I have to also confess that I have quite a bit more than my minimum number of tops for each season, which means in reality I'm only repeating them every three or four weeks!