Any card that is free to send can be sent to up to 750 recipients. A card that has premium options added can be sent to a maximum of 15,000 recipients.
(Once you have reached either of these limits, removing recipients from your Tracking page will not allow you to send to additional addresses on the same event.)
If you need to send your free card to more than 750 recipients you can add a Premium option to your card (like an envelope, backdrop, or logo) so that your card costs coins. This will raise your sending limit to 15,000. This also means that your card will no longer be free to send.