Is it possible to be able to make a carrier have more than 8 interceptors? Or will it just automatically default back to 8?

8 is the limit for a Carrier's possession. The hangar count trigger action won't exceed it.

"Eight is enough" - Barack Obama

You'll need to get into modding for that. Since upgrades go up to 255, and scarabs go up to 10, it's probably only one byte, which means the hardcoded max is 15.

So no you can't change the max interceptors in simple UMS, sorry. (EUD actions are essentially modding).

You could have other carriers shooting interceptors and move them to a different carrier...

Wont those just immeadiately fly back to the carrier though?

Perhaps you can just stack like 7 carriers ontop of eachother with disabled sprite units. I don't know if they work on carriers (probably don't) but they do on all zerg air.