Yoga blocks are often used in pairs. Occasionally you will use a single block for support.
At minimum you will typically want to own two yoga blocks. Some individuals like two sets, one set of soft foam rubber yoga blocks and one set of harder cork blocks for different parts of their practice. The soft foam rubber yoga blocks are often used for support, like under the sacrum in legs up the wall, while the more rigid cork yoga blocks can often be more useful to support standing balance poses like a standing figure 4, transition into an arm balance.
Each yogi's needs vary but I would suggest starting with a pair of foam yoga blocks.