There is no limit on how many links you can add. There is a limit on how many a visitor will read, which is the real constraint — the first two or three get almost all of the clicks.
- Open the card editor and find the Links section.
- Add a link with its title and address.
- Drag it into position. The order in the editor is the order on the card.
- Save. The change is live immediately.
Your analytics show which links are actually being clicked. If the link you care about is fourth and getting nothing, move it to the top and check again in a week.