When does SD-WAN make sense?
This is an important question, with a simple answer: it depends. And the main thing it depends on is, why an organization wants an SD-WAN in the first place. Answering that goes a long way to answering the size question.
The baseline assumption is that the IT department sees a need for the organization to have a iprivate WAN, rather than every site communicating solely over the public internet.
This is not a trivial assumption any more. As little as a decade ago, it was standard to have a private WAN for even two or three locations, since they would most likely be sharing back-end services of some sort from a common data center. Today, no such assumption can be made. Many companies grow to have many sites without needing private connectivity among them because everything they do is hosted in one or another external cloud. And, as some organizations migrate services out of data centers, they find that they need private WAN links at fewer sites or only at their data centers.