Here are a few categories I have five or more of (all are vintage or antique):
-- alarm clocks
-- electric fans
-- green or yellowware stoneware bowls
-- brown transferware, but I have been known to keep misc other colors
-- dog miniatures. A collection my mother started. I only keep one if it reminds me of her collection.
-- Ozark touristware pottery-- old leather Bibles or Prayer Books
-- enamel coffeepots and other vintage coffee items -actually my husband's collection.
-- porcelain doll heads. yes, just the heads.
-- hand made pig cutting boards
-- old tweed suitcases
I have a few rules about adding any item to my collections. I have to love it. I have to be able to display it so that I can enjoy it every day. I don't allow myself to pay "big money" for anything, it has to be cheap.
I don't even feel tempted to buy online or even in antique malls to find additions to my collections. It has to be something I just happen to run across at a sale or an auction.
Except for the rare small items (photos or dog miniatures) I have to get rid of one thing if I add something to the collection. And did I mention that I have to love it?