Yes, it means it was tested with up to 48 nodes.

There's no hard limit on the number of nodes, but at the moment BDR uses full mesh topology (each node has connections to all other nodes), which becomes an issue as the number of nodes increases.

Possibly, it means that they have tested it with 48, and any more is left as an experiment for the daring. Just guessing.

It's good to know what range people have tested with, even if it's not a hard cap.

I guess it means they reached production stability on 48 nodes, but there is nothing keeping you from adding more nodes if need be.