Quick question, as I'm new to the game.

When you spend a focus token to modify attack or defense dice, do you change all dice with the focus symbol, or just one die per token?

All of them.

Core Rulebook, page 11:

If the attacker has a focus token, he may return it to the action token supply to change all {Eyeball} results...

Not to be confused with evade tokens that add just one evade result per token. But you can use multiple tokens if you have them.

Thanks