In terms of the consensus algorithm, what's important is not how many peers have the transaction in their chains, but that the validators responsible for building the chain will not reverse the transaction. The certainty of a transaction rises with the number of blocks that are built on top of it. If your chain's mining-diversity is above 0.5, and a transaction is confirmed under (mining-diversity * number-of-permitted-miners * 2) blocks, then for all practical purposes you can consider the transaction to be irreversible. The only thing that could undermine this is a coordinated attack on the peer-to-peer network connections between the nodes – a level of attack that would cause any consensus-based system to fail in one way or another.