One or two networks might be OK to start with so that you can learn what you're doing. Later though....
Aside from the safety issues, having several networks worth of offers/products to choose from gives you a greater chance of finding something that you can successfully market. Idealy you would want to use a rotator script to test several different offers/products in the same niche with a patucular traffic stream. Again, the more networks you are affiliated with, the more options you will have.