Awesome question, Warrior!
The maximum upgrade level will always depend on the card's rarity.
Since each rarity is balanced to be equivalent to the other rarities on another level, all maxed out cards are in the "same level" balance wise.
Common cards
Common cards
Max level: 17
Rare cards
Max level: 14
Epic cards
Max level: 11
Heroic cards
Max level: 7
See you on the battlefield!
Over.