I usually have a pretty high poly count in most cloth I make these days, since most users have faster computers than in the past to make up for the difference.  A single garment (like a shirt or pants) usually have between 8-15000 polys. A dress can have a bit more than that.  

There's also more to it than what type of polys you use. Another importan factor is where the details are focused. 

I found that a shirt needs extra polys around the breasts, and under the arms. It doesn't need as much detail at the back (Unless it got modelled details like straps and such) 

If you look at the freeitem shirt I made earlier in the post, it shows a screenshot of a highres version. It's detailed enough in the right places to even allow draping around the nipples properly (Which most dynamic cloths don't. )  It's done with quads, but may have needed fewer polys if I had optimized it to triangles?

Pants need more details where the hip meets the thigh, and around the knees.