Hi @Shadow ,
Yes, the DHCP Relay configuration is applied per interface. https://knowledgebase.paloaltonetworks.com/KCSArticleDetail?id=kA10g000000ClFXCA0
If you have a LOT of sub-interfaces, you could configure one interface in the GUI. Then check the config in the CLI, and use text edit wizardly to quickly apply the commands to the other interfaces.