As a rule, assuming normal cut bacon, I find 4 rashers is the optimum amount for a good bacon sandwich - it provides sufficient bread coverage and sufficient bacon to taste and make the sandwich worth while.
My housemate, however, insits 2 is all that's required. As I see it, bread with a hint of bacon.
Doesn't only apply to bacon sandwiches, you need to have plenty of filling in a sandwich or its not worth bothering with as far as I'm concerned.
This thread seemed a good idea at the time.