Paul:
https://trac.torproject.org/projects/tor/wiki/TorRelayGuide#Parttwo:technica...
uses "ORPort [IPv6-address]:9001" - so can i use the same port number for IPv4 and IPv6?
yes
If I run more than one instance on the same machine, can i use the same IPv6 address like in the first instance - as i can do in IPv4?
yes