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One round is at most x5, so 2 rounds are 25, 3 rounds 125, 4 round 625...

(it's a bit more complicated and the details of the transactions matter, some people will send multiple utxos, some people will mess up after, etc)

Yeah there are x5 outputs in a mix, but there are also 5 inputs. At least 3 inputs, but up to 4, are premix and at least 1, but up to 2 are remixers. So I don't think it's as simple as your math.