I’m the same, so if you’ve completed your Piplup checklist and want someone to verify it, lmk. 
Apparently I’ve made a Piplup list for a collector back at the start of 2019 (which no doubt contains some errors, and is defintely outdated). I make lists like that pretty often for single Pokémon collectors like yourself.
This is also a very useful thread for all English cameos (unfortunately we don’t have a similar list for Japanese artworks yet), for which there apparently are 8 if I do a quick Ctrl+F for piplup.
Welcome, and enjoy your stay. I’m looking forward seeing your collection thread pop up eventually.
Greetz,
Quuador