How many computers do you need to build a network?

Could you please elaborate on the specifics of building a network, and why the number of computers needed is a crucial factor? Are we talking about a simple peer-to-peer network or a more complex decentralized system? Does the type of network, such as blockchain or a traditional server-client model, play a role in determining the necessary number of computers? Additionally, what factors should be considered when deciding on the number of computers, such as scalability, security, and maintenance requirements?