I use 9. 5 at the back pile and one at each over pile. 2" white with no scent. This is a minimum. If you want add some additional to the back pile so that if you have any issues you can hammer them out without having to re bird.
I put out a BUNCH at the back pile, 2 dozen or more combination of colors. I don't want to run out and I know I'm going to have to pick some up when through. However I am continuing on building momentum to that back pile. I want their default judgement to dig BACK
For the over piles I put three at each one, making sure they are white for suction
No need to use scent as you want to do it in short grass so the the dogs should be able to see em.
I never scent bumpers anyway, contrary to what some will say the dogs smell bumpers just fine
I agree with Ken twice the number to the back pile, and I use orange bumpers for my back pile so
I can keep track of the number left in it, as I've been taught sending a dog to an empty pile is a no no. I don't use scent as I want my dog to use his eyes not nose until he gets close enough to the pile. I'm sure he smells the bumpers any way, but not so sure they see orange. If I want to use a scent I will salt the areas around the centers or sides of the T. That way I can teach the dog to follow my cast past the drag back. Make sure your dog has the double T concept pretty well down before you use that one as it's another lesson for them to learn.