Depends partly on how much data each device needs to transmit/receive, and how often. Theoretically, rs485 can multi-drop a large number of devices. I have used rs485 almost strictly via modbus protocol, but I see you do not mention modbus.
Using modbus rtu (rs485) with the mb2hal component, I was able to operate 1 VFD per converter/adapter. I was reading/writing approximately 16 parameters per VFD several times per second. I could not reliably run 2 VFDs on one converter. It seemed I was pushing the limits on bandwidth. But I had it running a lot slower - I think it was 9600, I needed that for better interference tolerance.
I used the ebyte NA111 ethernet bridge. They seem ok. Maybe a bit sensitive to interference. (I was using them in a large cabinet with one 30hp, one 10hp, two 5hp, and one 3hp 480VAC VFD, so there was plenty interference available and they've been running relatively reliably since about 4 month ago when they started using it.)
I also tried using the ebyte NE2-D11 but was very disappointed with it. It was so sensitive to interference it did not work with the Allen Bradley VFD I tried to interface. Well, it worked, but froze every time, the instant the VFD was started. I want to try it sometime on a sensor, or maybe even multiple sensors, I expect it might be fine for that.
I also have on another LinuxCNC installation a cheap USB to rs232 converter reading a position sensor about 50x per second nearly continuously since about 2016.
