For security reasons, Venmo places a limit on the number of cards a user can add to their account. Users can have up to 6 active or deleted cards in their account at any given time over a 6 month rolling period
The 6 card rolling limit means that once you add a card to Venmo, it will count against your card limit for 6 months, even if you delete it. So, a card applies to this 6 card limit if it is was added fewer than 6 months ago (even if it has since been deleted.)
If you are seeing an error message that you've reached the limit, you are likely hitting the rolling limit and will need to wait until it’s been 6 months since you first added a card. At this time, Venmo is not able to increase your limit above 6 cards.