When putting a listing Under Contract, you can only enter 2 purchasers.
You can, however, add additional purchasers to the listing once the listing has been moved to Under Contract.
Once the listing is Under Contract, navigate to the listing details page and find the PURCHASER section. In the ellipses next to the last purchaser in the list, you will see the option to + PURCHASER.
The additional purchaser will then be displayed on the Edit Contract page along with the other purchasers.