How many people will there be?
Each trip has a stated maximum number of participants. For two instructors it's twelve. For one, it's generally seven or eight. That being said, if there's only one space left and you're a couple wanting to sign up, I'll stretch that by one for most workshops.
For some workshops or tours, maximums must be strict due to things like space in rental vans or available rooming. For some workshops where I know that if there are too many people that we'll all just get in each other's way at many locations, I'll be strict about maximums.