One question, answered from the books
Should I one-trick in League of Legends?
For climbing, mostly yes, with one adjustment: a one-trick plus one or two backups, because bans exist. Repetition is what converts hours into rank. Every champion you add to your pool divides your reps, and the data shows low-population champions hold far more Challenger and Grandmaster mains per game played than the crowd picks do. Pick your one champion from the list of who is actually at the top, then learn a backup for the games it's banned.
The decision, as a picture
What the numbers say
The one-trick data is unusually clear. The thin end of the popularity curve holds Challenger and Grandmaster mains at densities the crowd never touches. Rek'Sai shows roughly 180 times more high elo mains per game played than Malphite. The full ranking of all 173 champions is here, along with everything the data can't tell you.
The catch is bans. A one-trick with no backup donates every draft where the champion is taken away. The honest recommendation, then, is one main plus one or two backups that share the same job: same role, similar game plan, so the reps transfer.
The concept: Reps are the whole argument
Nobody climbs on champion knowledge alone. You climb when the champion stops costing attention: when the combos and the clear run on autopilot, the matchups hold no surprises, and your eyes live on the minimap. That state is bought with repetition; a pool of six champions never delivers it.
One-tricking also front-loads the painful part. The first fifty games on one champion are worse than variety, because you meet every counter and every bad matchup and you can't dodge into a different pick. Then it inverts: the matchups become known problems with prepared answers, and the map opens up. Most players quit during the painful fifty and call one-tricking a failure.
If you already know your champion, commit. If you're still choosing, start with what champion should I main and pick from the main density list.
"Popularity is not viability. Malphite is played 922,790 times and has one high elo main. Rek'Sai is played 86,741 times and has seventeen."
Best Champions To Main