Shunt resistor requirements.for 4-20mA current loops
The most common shunt resistance for 4-20ma current loop systems is probably 250 ohms, but 500 ohms and values lower than 250 ohms are also used. The most common supply voltage seems to be 24 volts. With a 24 volt supply, and 250 ohms shunts, up to 4 receivers can be connected in series with one transmitter. I doubt very much that there is a standard for this. If you can't find a definitive answer, look for web sites of manufacturers of commercial 4-20ma equipment and see if they cite any standards. Individual user companies may have their own standards.

Here is some reference material:

Murata PDF

National Instruments html