On StubHub, sellers can list tickets in any combination they wish. If they choose to sell the tickets in multiples of 2 or 4, you can't buy 1 or 3 of the tickets.
 

Finding a specific number of tickets on a desktop

1. On an event page, click All tickets
2. Choose the ticket quantity you want
 

Finding a specific number of tickets on the StubHub app

1. On an event page, tap Filters
2. Under Number of tickets choose the ticket quantity you want
3. Tap Apply