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Start with a diverse collection, see what grows well for you, and, most important, see what captures your imagination. You will find you like certain species more than others. Get more specimens of the ones you like. Become expert in one or two species. Your collection can be as diverse or as narrow as you want. There is no "official list" of tree species "allowed" for bonsai. Some trees work better than others. I have yet to see a "good catalpa bonsai" in my 40+ years of experience. On the other hand, Japanese Black pine and Japanese white pines work quite well for bonsai.
So what ever ignites your imagination. I like fruiting bonsai. Malus, especially crab apples are great for bonsai. Cotoneaster is good. Princess persimmon is not winter hardy in my area, but I have a couple, that I protect in winter. American persimmon is hardy, have a few seedlings, all less than 10 year old. Cherries, apricots, pears, apples, plums, blueberries, most culinary fruit species have some potential as bonsai.
The "right size collection" is one where you have enough trees that you always feel a little "behind" on your bonsai chores. This way, when you need to let a tree grow for a year or two, you will have enough other trees needing attention that you can set one aside and let it grow. For some the "right size" collection is a dozen trees, for some 200 or 300 are not quite enough. Much depends on how much time you have, and the stage of development of your tree.
A bonsai tree that is near exhibit quality will require several 4 or more hour sessions of work wiring or pruning or grooming every year. Young seedlings that need to grow out just need watering and fertilizer every now and then. A collection of seedlings takes very little time. A collection of mature exhibit ready trees, each tree will require many hours of attention at least once or twice a year, plus the usual watering and fertilizing.
View your collection as dynamic, you have trees "coming in" and trees "getting moved on" to other owners or worst case, the compost heap & burn pile. Nobody kills trees on purpose, but occasionally things happen. There is a learning curve. There are distractions in life. Your doing good if most of the trees "leaving" are "sell or trade or gifted" away.
And if every seed planted were to live to maturity, the world would be over run with trees. There are times when excess seedlings need culling down to a select few to move on toward bonsai. Can't keep 'em all.
So a specialty collection, or a broad general collection is up to you. No one person can be expert in all species and styles of bonsai. In time what you like will become apparent to you. Focus on what you like.