What is the minimum number of block confirmations required after which a transaction won’t be reversed?
Unfortunately there’s no single correct answer to this question.
Finality is probabilistic with our current consensus algorithm. This means that transactions can be, in theory anyway, always be reversed.
I am going to assume that you mean intentional reversal by a malicious party. The following also assumes that no single miner/pool has more than 50% of the hash rate because then they could just be bribed.
So the answer to that question depends on how much security you want. Security here would correspond to the cost of reversal, which would roughly be the cost of running a 51% attack, i.e. renting/buying the hardware required to acquire the mining power.
If we assume that it costs 5000USD to reverse a single block and you buy something for 50USD, then no rational attacker should do a reversal and you could consider it safe as soon as you see it included in a block.
If instead you buy something for 50,000USD, then you might opt to wait for at least 10 blocks.
The full picture is a bit more complicated than this but the above should give a decent intuition.
And I hope the explanation is sufficient as a reason as to why I’d rather not just give you a single number.
To get a feel for some theoretical attack costs, you can check https://www.crypto51.app/