To complete your collection, look for Expansion Packs - Series One (18 pins).
Some pins will be harder to find, so trading among your friends will increase your chances of completing the entire collection.
Pins to collect from the first release:
Yugi Base Set - 3
Kaiba Base Set - 3
Expansion Set - Series One - 18
Be on the lookout for pins from future releases!
Each Base Set Contains:
2 Common Pins
1 Rare Pin
1 Duelist Strap
1 Collectors Guide