In order to choose your barstools, you need to carefully measure your counter or bar in order to select the appropriate number of stools and the proper height. The diagrams below should help: 

DINING HEIGHT A dining table usually stands between 28" and 30" high. A dining chair whose seat height is between 16" and 18" would be required. However, you could also use shorter backless or swivel stools that measure 18" high.
COUNTER HEIGHT A lunch counter usually stands between 35" and 36" high. A stool whose seat height is between 24" and 26" would be required.
BAR HEIGHT A bar usually stands between 40" and 43" high. A stool whose height is between 29" and 30" would be required.
SPECTATOR HEIGHT Extra tall tables and bars are usually between 45" 48" high. A stool whose height is between 32" and 34" would be required. These heights are typically found in commercial settings.
HOW MANY? When the widest part of the stool extends between 16" and 18", you must plan on a stool width between 21" and 22". When the widest part of the stool extends between 19" and 22", you must plan on a stool width between 24" and 25". Swivel stools or stools with arms will require even more space.