The number of nodes per subnet on the Internet Computer is dynamic, expanding as more nodes come online. Refer to the Internet Computer Dashboard for more information.
The NNS governance system (which is governed by the votes of neuron holders) decides how large a given subnet on the Internet Computer will be.
The continuous goal of the Internet Computer Protocol is to maximize node ownership independence as well as geographic variance.
One of the Internet Computer Protocol's key features is resumability. This means that it is relatively trivial for a node to leave or join the network without affecting the liveness of a subnet or taking a long time. This is important for the success of the Internet Computer, since some blockchain projects decrease in the number of nodes as it becomes harder and harder to fully participate in a network. Resumability is also important for the performance of the Internet Computer, allowing it to resume a node without downloading the entire state, and to do it fast. For more info, read about catch-up packages as part of Chain Key Technology. There’s also a Medium article and video on resumption.