It really depends on the amount of free time for bonsai one has, and the level of development of trees. Your 8 to 30 is a good bracket for advanced, exhibition quality trees and only an hour or two a day to take care of them.
If you have a lot of "sticks in a pot", they don't need much time or care beyond water and fertilizer. You can easily care for 100 to 200 of these sorts of trees with the hour a day budget. The problem is, as these trees develop, they will require more time. At first we build up large collections of raw material just to keep ourselves from overworking raw stock. One cure for impatience is more trees. However as these trees begin to need more time, one needs to immediately begin to "downsize" the collection, by eliminating your "junk trees" in order that you have enough time to put into your better trees.
It is best to think of our bonsai collections as transitory. Trees come in, tree are passed on or sold. Occasionally a tree is composted.
Show quality trees require large amounts of time, you don't want to loose them because you neglected them in order to keep raw "sticks in pots" watered and growing.
Be responsive to changes in your trees. Be conscious of the need to downsize as quality of trees comes up. Be aware of demands on your time. Keep control so you can enjoy the hobby longer.