Stacy's got some good points: don't over-spend to start with, because dead trees are part of the tuition when you learn bonsai; do buy enough to keep you engaged and learning.
Try to figure out as early as you can just
what part of bonsai appeals to you the most. I don't see that mentioned very often.
For me, the creation part is definitely what has the most fascination. Once I realized that, I understood why, every time I bought a finished tree, I quickly lost interest in it! I now buy only raw stock.
Harry Greer, on the other hand, likes to own bonsai, admire them, maintain them; but he prefers to hire a professional to do the in-depth work, partly because he knows it's not his gift, and partly because he has the money to do so. (Don't knock him. Nothing wrong with having money if you're responsible with it, and I think Harry is.) So ask yourself what aspect of bonsai you still want to be doing when you're 89!
One other comment from personal experience -- OK, two.
First, "how many is the right number" does depend on many things: your own approach to routine, whether you like structure better than flexibility or vice-versa, other hobbies and interests, your physical energy, the money you have available. I'm half-serious when I say I doubt that Stephen Hawking could reduce it all to an equation! You'll have to explore what works for you.
Second, if you're like me you'll find out quickly that a wife needs and expects more of your time than you anticipated.

Her needs and wishes are valid, tho, as yours are to her. Be ready to work out a boundary that you're both comfortable with. If she loves you she'll be reasonable.