Some card numbers are not used in the game (or only in an expansion).
Each card has a unique ID number (on the back of the card, except for the Action cards – blue back – and Exploration cards – foggy back –, which carry their IDs on the front).
First of all, please refer to the complete list of cards to make sure you are dealing with a missing card: https://www.the7thcitadel.com/en/resources/downloads
Some cards might get stuck together, so make sure that the missing card is not stuck to the previous or the next card.
In addition, some numboxes (the icon containing an Adventure card’s number) are attached to a pictograph that means “take this Adventure card only if it is available”. If the card in question is not available, it is probably already in play, or has been discarded or banned, which would be why it cannot be found in the base box.
If you cannot find the missing card, please contact us using the following form, indicating which card you are missing, and in which language, so that we can send you a printable .pdf of the card: https://www.the7thcitadel.com/en/after_sales/