A capsule wardrobe is a method, not a number. The defining property is that pieces are chosen so they combine — a restrained palette, consistent formality, and fit you actually like.
Most people run between 25 and 40 pieces per season, excluding underwear, sleepwear, activewear and occasion clothing. The count is a constraint to think against, not a target.
The term was coined by Susie Faux, who ran a London boutique called Wardrobe in the 1970s. Donna Karan's 1985 "Seven Easy Pieces" collection carried the same idea into mainstream fashion.
Not to be confused with a minimalist wardrobe, which is about owning less, or wardrobe essentials, which is a list of specific garments.