This is a counter for "one flat object" or "one sheet." It is used for items like paper, stamps, plates, or clothing. This counter is versatile for various thin and flat items.

紙(かみ)、切手(きって)、お皿(さら)、服(ふく)など、薄(うす)くて平(たい)たいものを「一枚(いちまい)」数(かぞ)える助数詞(じょすうし)。