The 30 Pikachus
One guaranteed in every pack, thirty in the set, a different illustrator on each. Open the 30-pack Ultra-Premium Collection and, if they come evenly, you would still expect to be about eleven short. Finishing the thirty by opening alone takes around 120 packs, which is why people trade them instead.
Building the whole set? Track 30th Celebration as a master set and you get all 161 cards as one checklist, these 30 counted separately inside it, and an alert the moment a card you still need is listed for sale.
Track this set
Opened a box and got the same ones twice? Everyone has. One is guaranteed in every pack, so the spares pile up long before the set fills in: open thirty packs and, if they come evenly, you would still expect around eleven of these missing. That is what makes them the cards people swap. Trade your spares for the numbers you need, or list them and whoever is waiting on that illustrator gets told.
Prices are live market values, not asking prices. See all 161 cards in 30th Celebration.