It really depends upon your priorities and how much time and energy you want to dedicate to pinball vs. other hobbies and areas of life.

I can only speak to my personal preferences (short attention span, quintessential “jack of all trades, master of none” with regard to hobbies/recreation), and would vote for a critical mass of around 8-10 games. (This assumes you are not otherwise limited by practical considerations like space or spousal acceptance factor.)

That is the sweet spot to me; it is sufficient for a decent enough variety to stay interested, but avoids the situation where I become uncomfortably one-track minded and start spending more time “curating” the collection than playing games or doing other hobby/family things.

I had 20 games at my peak, then sold off half due to moving to a much higher COL area and needing to downsize the house; when I moved back to a lower COL situation I once again had the ability to house more games, but came to realize I was actually happier and more content with a smaller collection. I am now having a hell of a lot more fun than I once did, knowing that there is no longer any threat of pinball burnout looming over me.