If you're talking about Animal Crossing, then yes, you must have a Memory Card for each town or town backup. I am assuming that even though it would have the space for multiple towns, a Memory Card 251 would only be good for one town because the game will only save one town per Card. As far as I know, all of the Nintendo-published GCN games I have (six) will only save one "item" each to a Memory Card. Sonic Adventure 2: Battle, on the other hand, saves as many "items" to a Memory Card as it has room for.
I say "item" because I don't know what the official term for it is.