SimpleFoto
TPF Noob!
My experience so far ( I have around 200 images on various stock agencies) is that I don't think I will be able to give up my day job in the foreseeable, Not a bad little earner, and the most surprising images seem to generate the most money.
Most sites give you the chance to see which images have been downloaded the most which is a great guide to what's selling.
Good luck ... it seems to me , to be all about numbers.
Actually, I think it is all about quality. Generally 90% of sales comes from 10% of your portfolio. If you could upload more of the images that sell you wouldn't need a very large portfolio. Some photographers are able to do this (not me) and have great sales with a relatively small portfolio size. Kelly Cline (a very successful iStock food photographer) said at PhotoPlus something I love to quote. More images don't make me more money, more of the best images is what makes me more money.