The Line of Silent Guessers

Twelve prisoners stand in a single-file line, each wearing a red or blue hat assigned by an adversary. Each prisoner can see the hats of everyone in front of them, but not their own or those behind. Starting from the back, each prisoner (in order) announces a guess of their own hat color, audibly, with no feedback given. Using an optimal pre-agreed strategy, find the maximum number of prisoners who can be guaranteed to guess correctly.