Only need to promote units to rank 5 for this achievement!
36 of the same card is required (+ minimum 225 supplies (for Common units), depending on the unit rarity) to promote a unit to rank 5.
Rank 1 - starting rank
Rank 2 - requires 2 cards the same + 5 C / 50 R / 400 E
Rank 3 - requires 4 cards the same + 20 C / 150 R / ? E
Rank 4 - requires 10 cards the same + 50 C / 400 R / ? E
Rank 5 - requires 20 cards the same + 400 C / ? R / ? E
Rank 6 - requires 50 cards the same
Amount of Supply needed
C = Common unit, with Grey card
R = Rare unit, with Blue card
E = Epic card, with Purple card