Every deck is designed under a charter, and the charter is chosen before the first card is. It sets two things: how many cards the deck holds, and how much complexity may be spent designing them.
Complexity is a budget, not a score
Complexity measures how much a card can do. A card that does more costs more of it, and the charter says how much there is to go around. So a powerful card is never forbidden — it is paid for, out of the same budget as everything else in the deck.
That is the whole reason the number is shown as you design rather than checked at the end. A limit you discover on submission is a limit that wasted your afternoon.
What it buys
Two decks built under the same charter are the same size and were designed with the same means, so they can be played against each other and the result says something. Everything else about them — what the cards are, what they do, what they look like — is entirely yours.