Hi- I think I know the one you mean. Its title is "Numeral Correspondence" or something like that, and it was featured on one of the early Sesame St. record albums ("The Count Counts", I think). And I'm not 100% sure, but I think I recall seeing it in one of the Noggin episodes as well. Can anyone confirm?
Anyway, it is a cute clip with each of the muppets dressed as characters from operas or operettas. The first girl who comes out is dressed in a viking helmet, the last muppet is dressed as a ship's captain, presumably from one of the Gilbert & Sullivan pieces.

The opera begins with the girl muppet singing, "Let's sing a song of numbers, fa-la-la-lee, to tell us how many friends we see. . ." It ends with the five muppets on stage counting down from 5 to one and singing "Goodbye!" As the crowd cheers, they take their curtain calls.