Have as many as you have. In other words, if you have five styles of dresses, then your "dress" category should have five items.
Why do you think you need to limit the number of items on display? If you go to Amazon or any other large online retailer and browse through any given category, you will likely see dozens or even hundreds
of items. Having a big selection doesn't seem to have hurt them any.
If you start to get a lot
of items in any one category (say, 20 or 30 designs of dresses) you might want to give subdivide the category into, say, "evening dresses," "casual dresses" and "career dresses," or something like that, and give customers the option of using the subcategories -- but IMO you should still offer the ability for the customer to see everything in the main category without having to drill down into the sub categories. When I'm shopping, sometimes I like the ability to check only specific subcategories, but sometimes I want to browse everything. Giving your customers that choice will allow you to appeal to both mindsets.