If its strictly nymphs, not buggers or pupae (caddis or midge), then its GRHE in olive, natural and black...If that counts as only one then add PT and an Iso, swimming type nymph.
Actually its a silly question for me because I bet I have no less then 30 different kind of nymphs in my box and I hardly use any.
I literally use anything small and black in the spring and various colors and sizes of Hare's ears for everything else. I'm just as happy fishing a bugger in every fashion imaginable when nothing is rising.