Does the Attackbonus that you can get with the Wand Mastery count for the whole power (every target of a fireball) or just for ONE roll (only one target in that fireball)?
Yup, you roll an attack roll for every monster.. so only one gets an bonus.."Once per encounter as a free action, you gain a bonus to a single attack roll..."
Sound pretty clear to me.
It does seem odd that the class that is most focused on area effects, and thus most likely to have attacks with multiple attack rolls... has class features that only apply to one target.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.