Well, I have upwards of twenty different species, about about seventy total things-in-pots. And I've only been serious about bonsai for about a year! The problem is that a person has a hard time truly understanding the cultivation needs of so many species. (and I am stressing the difference between "keeping it alive" and "really understanding the application of technique".) Five species would be much better, with trees in different phases and perhaps different cultivars in each species. It just seems to me that you can learn more in depth technique if you focus and apply your time and energy to fewer species of trees. And you can be more successful if you have, say, five of each type of tree so that you can be willing to learn and make mistakes, or take the trees in totally different directions, and actually have something to compare them against. If you only have one gingko, for instance, then you can only apply one technique at a time. If you have five, you can apply different techniques and see which works.
Also, for ease of watering and fertilizing... so many different needs!! How do you know for sure that you are doing the best for them? If you have five of the same kind, you can vary the application and actually see results.
Just a thought.... I wasn't suggesting that everyone only go for one or two kinds of trees!!
Joanie