Well, one advice I've got that was decisive on my first year growing bonsai was "get 50 plants". I was amused and didn't want to believe that, assuming it was some sort of joke.
Then I killed my first plant, followed by a few others. TLC, of course.
Then I understood the advice.
Having a lot of plants will keep you busy all the time and rotating the bonsai in your care, the plants will have time to heal and rest from your abuses.
But...
It's not only the quantity but more important than that is the diversity. Pines are different from junipers that are also different from maples, tropicals, broadleaves and so on. The timing among many species is also very different: quince has to be repotted in the fall, pines when the buds swell, ficus are defoliated in the summer, many deciduous are wired in the winter.
So, having many trees of different species will keep you busy all year, and the plants will be happier.
They like to be left alone doing their own thing. Think about that and create new space on your benches to accommodate the new bunch of friends.

Clicio wrote: It's not only the quantity but more important than that is the diversity

Not what you intended to write, but yes... works for me - survival of the fittest.
I now know that certain plants just don't do well in my garden. I have had quite a few maples, in my garden, in pots... they don't do well, or do well for a few years and then suddenly die. Acer Buergerianum does seem to work out though.

Azaleas... not sure yet if they're going to be happy with me.

My pines are doing fine - but I've killed quite a few junipers.

Always a good idea to include native trees. They are all doing well.

Auk wrote: Always a good idea to include native trees. They are all doing well.

My experience too.
Natives are the easiest to care for, and the healthiest of my trees.