Great question here!
I feel that initially this depends on one’s goals. For gigging and touring musicians it’s much more important to have an extensive, well-memorised repertoire than for someone who plays for personal enjoyment.
In my seminars I often discuss the breadth vs depth of repertoire. For example see the 6th chapter here.
Both of these are important. By working on the breadth of our repertoire, ie. the number of tunes we have learnt, this exposes us to lots of different harmonic and melodic situations and also allows us to draw similarities between tunes, which overall makes them easier to remember.
In my opinion, the more tunes that we learn, the easier it becomes to learn the next ones, and also the easier it becomes to pick up any lead sheet and arrange it on the fly.
As a guess, I have perhaps covered 60 or 70 tunes in PianoGroove lessons and seminars. Can I play all of these arrangements perfectly from memory? Absolutely not, but I do have a working knowledge of the tunes and the harmonies should I wish to revisit them.
Now to discuss the depth of our repertoire. I have perhaps 10 or so tunes that I play on a regular basis, these are the tunes that I spend a lot of time listening to and transcribing from. They are my go-to tunes tunes when I want to try some improvisation or reharmonisation. For now this number of tunes keeps me busy and gives me enough material to explore jazz piano. I’m sure this number of ‘in depth tunes’ will expand in the future but for now it works great for me.
Personal and professional commitments can also be a big factor. For someone whose job is to play music, they naturally have more time and opportunities to practice and memorise tunes. In which case there is no problem to shoot for a vast repertoire.
For someone who has to balance learning music with family and work commitments, it’s perhaps better to focus on a smaller number of tunes and learn these to a high level.
In summary working on the breadth and the depth of your repertoire both have their advantages. I feel that exactly how you split this is based on your musical goals and this will be different for everyone and so there is no set-in-stone answer here in my opinion.
I hope that’s helpful!