[arm-allstar] multiple nodes on same public IP address

[C B]

Hi
as part of the replacement of my controllers and migration to allstar with PI3 nodes i am also migrating to a router using one public ip address.
in order to do this i need to assign a new port to each node sharing the same public ip addresses.
is there  a recommended range for the ports? 
is there a limit to how many ports i can use with the same ip address?
there could be more than 20 ports if i use one ip address.
Chris