A sparrow is a sparrow. Two sparrows, I guess, "two sparrows". What about a... flock, though, a gaggle or a flight?
Sparrows usually come in "hosts", it turns out. Now, if you want to collect not nouns and sparrows but email inboxes in Sparrow, here's the host of actions to take:
›› Don't go inbox-hopping in Sparrow. Have it collect the incoming messages from all your accounts in one place instead.