What Dave says makes sense and also does seem to work.
There isn't any point in having a class which no-one (or very few) support.
Open class, fine. Most people shoot in that anyway.
Juniors, excellent. We have to have this class to encourage youngsters into the sport.
Springers, again if supported by entries it can be run.
Any other class surely has to be justified by the number of entrants.
Perhaps a percentage number justifies whether a class is viable, say 10% of entries. Juniors should be exempted from that requirement.