How many nodes can we have in a DDS domain?

You mean that IP multicast cannot technically manage so many nodes?

I think it really depends on the pattern of subscription. For each Topic there will be a number of DataReaders and a number of (matched) DataWriters subscribed to that same Topic. This what we call the "fanout".  The typical fanout for each Topic is much smaller than the total numer of nodes because not all nodes subscribe to the same Topic.

I do not think multicast has a problem having a large fanout, quite the opposite the bigger the fanout the more benefit you get from using multicast versus unicast.

Keep in mind that Connext DDS does not use a single multicast address for all traffic. Rather it is possible to configure it such that different Topics use different multicast addresses/ports so that only the nodes interested in the Topic will get the data. It is even possible to configure it with even finer grain. Even with in a Topic you can configure content filters so that the data that passes different filters uses different multicast channels (see MULTICHANNEL Qos Policy).

We have seen many systems with more than 1000 nodes that operate without problems. But as the system grows there can be issues with scaling the discovery data.

If the system has too many nodes and you get into discovery or scalability problems, then you can use the RTI Routing Service to deploy it in a layered manner (following the IIC Layered Databus pattern).

Can you describe a little more what your system will look like? That way I could suggest somethign more concrete.

Gerardo