I was not aware of a hard limit, so I checked MSDN and couldn't find one there either. Obviously, more nodes puts more strain on your infrastructure, so the real limitation will depend on your specific server hardware and network.
For more information ...
Peer-to-peer Replication (Enterprise only): http://msdn.microsoft.com/en-us/library/ms151196.aspx
Merge Replication (Standard or Enterprise): http://msdn.microsoft.com/en-us/library/ms152746.aspx
I also found this limitation for Workgroup Edition, if you are using that:
answered Nov 05 '09 at 12:19 PM
Plus one for Toms answer. Don't forget the type of system you are running too. Peer-to-peer replication is most suitable for scaling out read operations. Because all writes must be replicated to all nodes in pretty much real time, a highly transaction system can cause load balancing problems. This should have been a comment really, but I can't do them yet.
answered Nov 09 '09 at 10:10 PM