It's a log, see fig here :

When the number of nodes increase, the 'reward' to be a node is less attractive AND being a node is less needed. It's also a normal phenomenon, in the beginning a lot of people want to be part of something, then when massive adoption has occured you have less and less people adding. The number of dash mined or transfered have nothing to do with nodes, nodes are just people who want to make a crypto live, for wathever reason. What can happen, because we are not in a massive adoption yet for crypto, is something like this :

So one can have any kind of function similar to a log (lot of grow in the beginning, then less), then a new phase of mass adoption, similar to an exponential (growing slowly in the beginning, then more), and again a new log phase, the whole thing being a big kind of log with some oscillation in it.