CDN nodes are usually found in different locations. This allows to narrow the costs of traffic, to make the page load times quicker as well as to increase the global availability of content. The number of nodes and servers in every network is different and depends on architecture. The queries for content usually algorhythmically is sent to nodes via optimal route. During the optimization process the nodes that can serve better to give content to end users are chosen. Usually those are the nodes that can give better performance. It is so to optimize the content delivery. If we speak about the cost optimizations the locations that are cheaper may be chosen, however they may be not the best in terms of route. Ideally there should be balance between price and productivity for each project.
Most providers offer set nodes number according to the desired coverage, for example - USA, different global packages, Asia and so on.