If a ship has more than one of a particular type of defense token, and the attacker uses a single accuracy result against one of them, does it prevent the defender from using any of that type or just the one?

Just the one. You can freely use the other. Each accuracy targest one specific token, not a type of token.

Which is exactly why some ships have seemingly redundant tokens, because you can't spend 2 of the same token at the same time anyway... Having the second is a backup for the first if its used or blocked with an Accuracy Token.