AFAIK there's no explicit limit per se - the BDR developers tested it with 48 nodes, but it will work with more nodes.
But there are things you'll probably run into - for example this only supports full mesh topology, i.e. each node speaks to every other node. So the more nodes you have, the higher the number of management connections.
But there are things you'll probably run into - for example this only supports full mesh topology, i.e. each node speaks to every other node. So the more nodes you have, the higher the number of management connections.