From the blurb on the website about the battery charger...
Each Apple Battery Charger comes with six NiMH batteries two for your keyboard, two for your mouse or trackpad, and two for charging.
Hmm, well my Apple wireless keyboard takes three AA batteries which has proved to be a PITA as all chargers I've seen can only take 2 or 4 batteries so this one will be no exception - or has Apple changed the wireless keyboard to only use two batteries? 😕
EDIT: Just checked on the Apple site and the current keyboards do take two batteries. The ones in the refurb store (where I bought mine) take three