Affiliate Marketing Secrets

I am interested in getting into this. My question is, let’s say you have 20 little niche blogs, do you have to get separate domain names for each? I won’t have much money to start with, but do want to start out doing it in a way that will have the best chance of success.

I mean, let’s say there are stock market investors. Is it a good idea for you guys to be constantly telling new people, won’t that saturate it? I don’t understand how people all over, on places like this can just tell others their methods. It doesn’t make sense, i’d keep it secret. Does the stock market experience saturation if too many investors join? I’m just wondering, I do not know.

Let’s say 2.000 users of a specific niche?