It really depends on who you play and which role.
Personally I have 1 or 2 champions I can confidently carry a game with for each role.
As and ADC main, I also have 3-4 champions to play ADC for when my main gets banned. That being said, I can play Xayah 80% of the time, so really you only need to know one other aside from your main.
as for deciding who to play:
If you are looking for new champions to play, pick ones with similar playstyles to the champions you like.
Lets go off Cait for this,
She loves to bully her opponent in lane as safely deal damage.
For another ADC why not play Ashe or Varus, they both are traditional marksmen that will sit back in safety and have powerful ranged abilities.
Now lets move to support, which is the easiest to learn as an ADC main since you already know the mechanics of the lane itself.
Going off the similar safety, lets try an enchanter like Soraka or Lulu, who can poke from range and have multiple ways to get their ADC out of trouble
I could go on but I think you get the idea, I would reccomend moving to PvP asap though.
Its very different to bots in the way the game is played but all the more fun.
Just a warning, blind pick is full of trolls who will ignore what roles others claim. Oh... you will also encounter plenty of smurfs for your first 10 or so matches so dont be discouraged if you end up doing terrible.