When you gain renown you gain card packs, roster slots, and heroes. Renown is an indicator of how many times you have leveled. It is not an indicator of overall strength or where you should be placed in pvp
If you have 6 characters sitting at level 5, or any other level, and you level 3 of them up 8 levels you gain the same renown as if you leveled all 6 of them up 4 levels, or If you had 8 heroes and leveled them all up 3 times.
You can’t tell someone’s strength just by their renown level. None of my characters are above level 13 and I am 5 level ups from being max renown. Having bought Jarlaxle helped because I have leveled him 10 times so far which is about 1.5 levels of renown.
I did not pay attention to the early levels when it came to how many level ups to get a point of renown, however I do know for a while, at least through level 14 and 15 of renown, it was no more than 6 levels and now it is 7. My guess is the earlier levels were less than 6.
I would bet that it is actually less, but just for ease of math, if you say it is an average of 6 hero level ups per level of renown then to get to level 20 it would take 120 level ups. Also, it is actually probably less than that. I think you actually start at level 1 renown so you only have to level up 19 times. I might be wrong and you start at 0 though, with 9 heroes that is 13.333 levels per hero to get to level 20. 12 if you have jarlaxle.