My first response was quality. As Della says, why would you want to train a response you do not want.
Then I thought a bit more - and thought of some scenarios where I might go for quantity.
If I had a horse who was very forward and perhaps over reacted to aids, I might plan to do frequent transitions at various pre-determined points in school to get horse listening to me and engaged in listening and responding. Once horse was going up or down a gear when I wanted without over-reacting, I would start focusing on quality of transition. I'd almost be looking for this type of horse to start thinking upward transitions were boring so they didn't over react. (Obviously I'd use my judgement and look for another solution if I wasn't getting expected response).
For a horse at the other end of the spectrum, slow off leg, or switched off, frequent transitions would start to engage them, and I'd almost be wanted to get them over-anticipating. Say if I did a 20m circle with an upward transition on quarter point and downward at next etc, I'd hopefully get to the point where they were offering to go up a gear, or down. And again once I got the response I wanted I would look to improve quality of transitions.
Like Erin's Mum I had Carl Hester's ideas about quantity of transitions in my mind. And his transitions are always about moving forward the horse's training in a correct way.