I have read a few people wishing to get down to a more respectable size and take care of fewer and better trees. That is good for a while. I have done that two times in my life and each time I find myself looking for that next best tree. You can weed down to say best ten. After a couple years entering the same five trees in the club show and the overkill on discussion forums with the same trees over and over again, it leaves one looking for something new to work on. One needs to have about 15 good trees (ready to exhibit in good pots). About 15 good trees in training (almost ready for a good pot). About 15 good trees as material being grafted, major wire and developing nebari. Any more than that is not enough time to work on properly. For me I count 4 shohin per medium size tree.
I tend to agree with Al here and of course it is all subjective. For me the best parts of bonsai are developing good trees...once developed they are less interesting to me. I enjoy showing trees and for me that experience is maximized when I am responsible for every stage of the development. I find it takes numbers to do this most effectively.
For example....about 6 years ago I found some large chinzan azalea nursery stock. I purchased about 40 of the plants over time. Drastically pruned and began developing all of them. After a couple years I began to sell the ones that appealed to me least. 6 years later, I have 3 pretty nice chinzan bonsai...and I'm likely to sell one more. Had I only purchased 3 to begin with I can assure you that they would not be as nice now! I could tell the same story with my collected bald cypress, my tridents grown from seed, boxwoods, and Shimpaku grown from cuttings.
We all have a unique set of life variables and goals that determine our personal paths in bonsai. So, whether you are a few trees kind of person or one who has many, the beauty of this hobby is that you can choose the path that best solves your equation.