We have successfully tested around 100 mesh nodes in a single network 1 year back itself and I am pretty sure that the number of nodes in a single network can be go much higher than that because the mesh supported chips have enough RAM to handle the data.
As per the spec, SIG says, mesh supports 32767 Elements in a mesh network. But a device can have multiple elements also.
In my opinion, limitations can be in the mechanism of dealing the flooding mesh data, cache list, other mandatory security procedures, etc which are more related to the software implementation. From the code I could understand the mesh cache replay size is 200, which means the device can accept messages from 200 different nodes and process them. But IV update procedure can clear the list and allow new devices to be part of the network communication. Also, maximum number of network keys, application keys, scenes and scheduler events are configured to 4, 8, 10 and 16 respectively as of now. So you can always create subnets and add more nodes into it.
Let me work with our software team and see I can get you a theoretical number of nodes which we support.