When I first started doing stock photos, the minimum submission was about 200 great quality pictures with regular "top up's".
Dunno however what the rules are today. It doesn't pay that much, as you have to have lots of really good stuff for people to see. The trick is to take "mundane" stuff that people could use to illustrate an article etc.
Good sellers believe it or not are subjects like street scenes, people in cafe's, restaurants, park views etc.
You could try taking a look at iStockphoto
to see what sells and what doesn't.