You have to match the number of tickets put up for sale by the seller. Sometimes the seller may request that all tickets are sold in one order, other sellers may be happy to sell (for example) 4 tickets one by one.
Also, you can only buy from one resale listing at a time per transaction. Therefore, there could be tickets listed for resale next to each other, but listed by different sellers. In this instance, you will need to make two separate bookings