This is a general counter for seven unspecified items or "seven pieces." It is used for counting various objects that do not require a specialized counter, like "seven books" (referring to individual volumes) or "seven colors."

形(かたち)や種類(しゅるい)を問(と)わず、物(もの)を「七(なな)つ」数(かぞ)えるときに使(つか)います。本(ほん)七(なな)つ、色(いろ)七(なな)つなど。