Q: Is it possible to put too many decoys into a pattern?
A: In theory, no, but in practice, you reach a point where every pigeon that enters your field, responds to your layout, rendering it unnecessary to add any more decoys. Beware also, that birds will begin to turn wide if there is not room to land safely in the pattern, and that constantly scurrying about setting up decoys, could disrupt the flightline.
There is only one reason for adding shot birds to the pattern, and that is to get birds to respond better. Once that goal is achieved, no further fiddling is required.