5 players (2 in front of me 2 behind) called the blind to the flop. I'm UTG +3 with Ac4c with around $100

Flop comes 3c 6c 9h

Two checks to me and I bet $5, one caller behind me and the big blind calls
(Both players have me covered by a good amount)

Turn is Kd

Everyone checks

River is Kc

Big blind bets $12 I raise to $40. Guy behind me folds and the big blind shoves. I called. Not sure if I should always be raising on the river with the nut flush or just calling with a paired board. Also was my $5 bet on the flop okay? Thoughts? I had recently sat down at the table so I didnt have a read on my opponents. The big blind was a young Asian guy if that makes any difference

with a 50bb starting stack, there's not much point in limping Axs unless you're willing to get it in here. Paired board is unfortunate but folding doesn't make a lot of sense, absent reads on opponents. BB can have Kx and filled up; maybe he was looking to check raise turn?

Limping pre is OK; without any reads on the table, prefer opening for a raise to limping. But limping is probably fine, esp if table is generally passive pre and you're unlikely to face a raise behind you.
Flop bet is too small. Half pot? A good result would be to win the pot here, so a decent bet -- more like pot sized -- would make more sense.
The turn check is probably a mistake, the K is unlikely to have hit your opponents and is a decent card to barrel.
Can't fault the river raise but when you get re-raised, that's trouble. If you were deeper, this might be a fold but, you started hand with $100; the chips are going in. V can have a lot of hands you can beat, including smaller flushes.

First, buy in for the max or look up a good short-stack strategy (limping A4s is not a part of said strategy). I'd probably fold this from early-middle position (I'd sometimes raise it in middle) and raise it from the cutoff or button, but only when full stacked.

Since you found yourself here, pot the flop, pot the turn, shove river. Sorry he had the boat (K9 I'd guess).

Limping A4s from EP with a short stack will not make money in the long run.

After that it's OK. River raise is fine and BB's shove is ugly but you can't fold the nut flush with that short of a stack.

Fold pre with only a half stack and fold river. Seems fine otherwise.

People aren’t often 3-bet shoving river without a boat.

Fold pre, bet more on flop, bet turn. River is what it is.

agree with every about a 50bb stack. The $5 on the flop is pointless to me. Bet more or check. $5 doesn’t accomplish much. I’d raise the river and call his shove with no concern at all.

Fold pre-flop.

Flop and turn are fine.

River raise is fine (hoping for a crying call from a smaller flush). Calling and hoping the guy behind also calls is also fine.

As played, I think you can save your remaining $53 once he ships it into you. This is almost always a boatm