I was a martial arts practitioner for a couple decades, and our answer was "add one at a time". You'd learn the entire system at a basic level, and then practice it all enough to figure out which one technique you wanted to be the strength in your personal flavor of the style. So you'd level that one technique up from just being able to do it, to knowing when and how to apply it to many different situations. Once it became an intuitive response, you started on another technique but tried to find a technique that would cover the gaps where the first technique was not practical.
So a newly minted black belt might have one or two techniques. 10 years later, they might have 5. 20 years later, maybe even a dozen. But it isn't a question of the right number of techniques - it is a matter of constant improvement. The goal is to keep practicing moreso than having a specific end point.
Competition time is 10 years max at the very high level with the usual few exceptions, and sometimes what was working stops to work, either due to injury, changes in weight or power, or competitors "figuring you out". For example, Riner was so physically dominant that as soon as he had the grip, it was flight time. Maybe he had a sasae too, I don't remember. Most players play "opposite" techniques, for example osoto-sasae is a classic one in judo.
For lower level/amateurs/"lifers" the math change, but at that level it is a bit more (or a lot more) about having fun, not destroy one's body and other concerns not related to high-level competitions, so more techniques can bee added to the arsenal.
I bounce between specialization and breadth because on the one hand, I want to "win" also in practice, on the other hand "win" what exactly?
Agreed, completely - I practiced chinese styles, which are far more about lifetime effectiveness and basic defense. It definitely changes the math of how much you can "master" when looking at it as a lifetime practice. It also changes the definition of what it means to "win", because our definition was that you were still strong with working joints in old age.
So a newly minted black belt might have one or two techniques. 10 years later, they might have 5. 20 years later, maybe even a dozen. But it isn't a question of the right number of techniques - it is a matter of constant improvement. The goal is to keep practicing moreso than having a specific end point.