JCI Engineering guidelines recommended
A maximum of 50 N2 devices if the supervisory controller is a N30.
A maximum of 100 N2 devices if the supervisory controller is a NCM.
They also recommend that you reduce the number of devices if any of them are VMA or DX-9100 devices. These devices require more bandwidth than the N2 Open devices.
The N2 protocol definition provides 256 addresses. 0, 254, and 255 are reserved for JCI use. So, you have 253 assignable N2 addresses. If you find a legacy installation with more that the recommended number of N2 devices installed you should consider splitting it into multiple N2 buses. Remember, you will soon be driving the legacy N2 bus from either OPC or BACnet transactions coming in at Ethernet speeds!
For more details on this topic visit the JCI web site and search for documents related to the N2 bus. They have some very good reference documentation.