Made to be a pair
Most beginners buy one block and quickly wish they'd bought two. Nearly every pose that uses a block uses one under each hand. Half moon. Triangle. Supported bridge. Seated folds. Practicing with two lets your body stay square, your hips stay level, and your shoulders stay open.
One block will carry you through the first few weeks of a practice. Two will carry you through the years after.
The single, for the occasional prop.
The set, for the practice you're actually building.