You don't need more than 1 bob for ANY race. The only reason to use more than 1 is if you need their physical strength to be greater than zero for most of the race (they recover while they wait in the pit). I've reached B-spec level 40/39 twice now and I only ever use 1 bob even in an endurance, so he gets all the experience instead of sharing it.
If you race the same car as another competitor, then the physical/mental state of your bob might be an issue, but the way most of us race B-spec, with superior cars, it's not relevant.