Every card has a certain number of charges. This is the amount of times the card can be played before the card goes on charge cooldown. The amount of charges is listed near the bottom of the card's artwork, in the middle of the card. The amount of card charges can be increased by charge upgrades.
Summoning a squad of units always consumes just one card charge.
If you spend the last charge of a card, the charge cooldown will be equal to half the power cost in seconds. For example, the Northguards card recharges in 25 seconds if its charges run out (50 divided by two). The charge cooldown will only add a charge if the card has zero charges, it will not add more until you spend that new last charge.
Most spell cards additionally have a spell cooldown that prevent a player from just spamming all their available charges at once. The spell cooldown will be listed at the end of the spell description. When you spend the last charge of a spell card, the charge cooldown is often longer than the spell cooldown is. In that case the spell cooldown does nothing.