I usually have my go-to prey distress, sometimes a quieter coaxer for calm or close-quarters situations, and a howler. I don't use a howler very much because the rabbit distress just works for me, so sometimes I switch that out for a different one or new one I want to try. More calls equals more clanking around noise and movement dealing with them while on stand for me. Mine are all open reed because I only hunt predators in below freezing weather and closed reed calls tend to freeze up.
Like Yotarunner said, I can make all the sounds with one, but some other options or a backup is good to have. Nothing worse than walking a half mile through the snow just to have one call and it doesn't work properly because it's dirty or something.