In Paris, a studio apartment is one room with a kitchen and bathroom. (We rarely offer studios, though there are a few exceptions - starting at 1200 €)
A "2 pièces" apartment includes a separate bedroom, living room, kitchen and bathroom/wc.
A “3 pièces” includes two bedrooms, living room, kitchen and bathroom/wc.
Each additional “pièce” usually means another bedroom, or it can refer to a separate dining room, a double living room, an office/den, etc.