I am a bit confused about what a good c-betting range could be in flops consisting of mainly low cards. To keep things simple, let's assume we are 20bb deep in the SB and min-raise a range like 22+, Qxo+, (J-7)6o+, Txs+, (9-7)4s+, (6-4)3s+

BB calls and the flop comes 632r. I understand we should c-bet a polarized range here:

As value-bets, we have our 66+ pocket pairs, 22, 33, all of our sixes and 54s. In total, 190 combos if I've done the math right.

Now the question is, what makes a good bluff here? How many combos should we be bluffing? (let's say for a 55% cbet size)

I understand that hands with a 4 or a 5 can in principle be good candidates because they have equity, but so do any two overcards. Hands with a 7 or 8 are blocking 76 and 86, so that's also good news, isn't it? Anyway I would still prefer to bluff with 97o than A7o (we may be ahead with our A7o and do fine by checking-behind, but is this resoning sound?)

By the way, how should my value-range react to the villain raising (maybe even shoving!?) How should the turn be played after a call?

What are your thoughts on this spot? Thank you so much!

Its probably easier to think about what hands prefer to check back.
You shouldn't be worried that your check back range will be pretty weak or what blockers you have so imo just check back hands that prefer to check back and bet everything else.
Board is pretty good for you, misses average flatting range and a lot of good cards to barrel to make weak pairs/draws fold.
What do you want to check back here? likely all 2x, most 3x, A highs, K highs, hands like QJ, QT, some backdoor hands like J8ss, 98ss etc
If you get check raised you definitely want to shove your value, board will change very quickly, even if he is bluffing with just 2 over cards, you dont want them to see turns

Pretty much sums it up!
It will definitely miss much of a flatting range at 20bb,especially if you are playing against fish,since their calling ranges pre-flop are more condenced towards middling hands like 9x,Tx,Jx,Qx.
I would definitely add a few more total bluffs that don't have significant SDV and check back some hands like the ones none888 mentioned above.
Hope it helped!