A maximum of 32 bus loads per RS485 port is a very common value in the industry. The driver devices on the board are specified for this number of bus loads and this can't be expanded by an external power supply.
In general it's a good idea to build multiple smaller networks instead of one huge network, as a single node failure could result in the failure of the whole network. If you want to optimize your cabling you may consider using ethernt based RS485 ports like the NI ENET-485/4.
I hope this helps,
Jochen Klier
National Instruments
