GR Yaris Flat tyre replacement - how many do I need to change?

[Beefstah]

I've picked up a puncture, and may need to replace the tyre.

Do I need to/should I replace the other tyre on the same axle? Do I actually need to replace all 4?

The set on there has done 12k, and has 5.5mm of tread left

 

[GRgrampa]

I would replace the pair on the same axle and keep the good one as a spare.

 

[Araf]

8mm is the starting point. My rule of thumb is to replace a pair if over half worn or if I cant get a matching brand and series of tyre.