I know in contrologix there was a connection limit for every processor.
Ethernet connection MSG was 1 connection.
HMI server sometimes have multiply connection per plc.
Total connections are in a manual.
I even think there is a limit in a SLC500 plc
If you look in the PLC5 instruction pdf I think it gives how much memory a MSG instruction takes plus the data table that your sending.
But PLC-5 I do not see it. I think your limited by if your msg works all the time without locking the processor up. So its a put it in and see if it works. Just remember PLC5 is not a fast processor, scan time 20-40msec usually is the fastest i have seen. Then if your doing Ethernet its better to use an enhanced PLC5. Not sure about the non enhanced PLC5.