I got this in the first mission of the Da Vinci disappearance. There's exactly 10 enemies. Oh and nobody has pointed this out (maybe it's because everyone knows it, but if someone doesn't) you can counter in the middle of a streak. If you see someone's icon flash while you are attacking someone else, you can hold RT and counter, but it's just best to attack the guy who is going to attack you next.
Nulla è reale, tutto è lecito.