South has 20-21 HCP and 2 spades. On occasion, 1NT bidders may choose to play in notrump rather than a major fit (on the similar auction: 

West	North	East	South
			1NT
Pass	2	Pass	2
Pass	3NT	All Pass

After a 2NT opening, though, responder doesn't have room to show extra shape with an extreme distribution (5-5, say). So we can put opener on exactly two spades.