You can add an unlimited number of links to a card.
To add a link to a card
1. Click on the purple button in the top left corner of the card
2. In the menu that appears, click "Link".
3. Insert the link and description (if you wish)
4. Click "Save".
The link will appear on the front of the card.
Please note, the link won't be reflected on the card front if you enable Compact view or set your personal one without the links. More information in this handy article.