Scalability is whether the tenth site costs the same effort as the second. Not whether the software technically supports a thousand screens (almost all of them do) but whether the humans running it are doing ten times the work.
The things that actually decide it are organisational. Can a new site be provisioned without a specialist? Can content be published to a group rather than screen by screen? Can permissions be delegated so head office is not the bottleneck for every local notice?
The caveat is that scale problems appear at a specific and predictable size, and it is smaller than people expect. Below about twenty screens almost anything works, because one person holds the whole network in their head. Around fifty to a hundred, the manual processes that were fine start to fail, and the platform chosen for a pilot is discovered to have no concept of groups, roles or bulk operations.
Evaluate at the size you intend to reach, not the size of the pilot. Ask to see a hundred screens managed as ten groups, with two levels of permission. If the demo cannot show it, the pilot will not survive the rollout.