It's possible to get infinite using the panning method, you can only no longer pan it when you've caught it and fulfilled the check for catching it (which removes it from the pool of available fish that the pan grabs from)It is also possible to get two legend fish; the wiki explains
If I'm not mistaken, you can only have 1 each of the Legendary fish, but you can have multiples of the Legendary II fish by doing the Qi Quest repeatedly.
Ohhh right, I forgot lolYou can catch unlimited amounts of the legendary family during the Qi quest. So it’s likely one Legend and 39 legend II ponds of someone has 40.