Korva, I don`t know about a "recommended" number of spawners in a world. I guess as long as a world`s every nook and cranny isn`t jammed pack with spawners, then I guess it is largely subjective. I say subjective as I am not really into gem/insect collecting, and first and foremost I have to please myself when I create a world. I know I cannot please everyone. I guess that`s just the way it goes.
I know that one of the gem spawners I added to Great Bear (near the mine at the top of Buck Falls waterfall) also generated snakes. I didn`t plan that it that way, but I kind of like it that way :)
