Well, there is little experience with Flutter systems, of course. We are all waiting...
In general - it depends on the quality of the system, the size of the network, position, topology, and amount of traffic, and the type of networking.
I do know of industrial systems who are getting 3000 of nodes working using 900Mhz radios. But they are using commercial software, tiered networking, and extensive tuning. They have multiple gateways that provide IPv6 connectivity from a central server to individual nodes.
Then there is the open-source OS that uses IPv6 and 6LoWPAN called Contiki
Of course - there is also the ZigBee technology which is not TCP/IP based.
They all have different characteristics.