What happens to me if I don't pin enough is that fabric will get flipped over and sewn in. And yes, you need a huge number of pins. From back when we first started with pins and used those big diaper sized ones, and now I have a more medium sized curved ones, I typically have a couple hundred safety pins. I keep mine in a cigar box.
I really don't recommend them but if you are going to do your project right away, maybe buy pins at the dollar store. You just don't want to use them for long term storage especially if you are in a high humidity area.
My preferred method is spray baste, but I'm going to be trying some basic Elmer's Glue in the near future. I was very disappointed with the bottle of spray I just purchased on a number of fronts, and basically I am cheap!