A pack of condoms
Guy goes into the Chemist's to buy a pack of condoms. Asked what size pack, he replies, "a pack of 10, that's what I usually get".
The chemist explains that they're now available in packs of 6, 9 and 12 only. Asked why the change, the Chemist replies that it's an attempt to meet varying requirements across Europe.
Customer: "How's that worked out then?".
Chemist: "Well you see, the 6-pack is mainly for Italians. They're very religious and usually abstain on Sunday night, so a 6-pack is a week's supply".
Customer: "What about the 9-pack?".
Chemist: "They're for the French, who are very passionate. Once a night Monday to Friday, and twice each night on the weekend".
Customer (with growing interest): "What about the 12-pack then, whoever would need that many?".
Chemist: "They're for the British".
Customer (astounded): "The British???, how on earth can that be?".
Chemist: "Oh, you know, one for January, one for February, and so on".