All of the tickets that you see listed for sale come from different sellers, who often have different delivery methods, estimated available dates, and of course, who need to be paid separately. For this reason, we cannot merge two listings into one order.
Additionally, you may find that you can't select the quantity of tickets you are looking for even if the seller has that many available. For example, if you are looking for 3 tickets, but you only see an option for 1, 2, or 4, it means that the seller specified that he or she did not want to be left with a single ticket.