He can understand that an event organizer needs to sell tickets in the form of precise multiples.
To better understand the concept of multiples of tickets that can be purchased, take for example a live show for Valentine's Day that also includes dinner. In this case the promoter of the event needs to arrange the tables by pairs or more pairs together and therefore the multiple to be indicated will be 2 units. Another example could be a course with lessons for groups of minimum 5 people.
Likewise, situations may arise in which the nature of the show and the optimization of ticket sales requires the purchase of a multiple of tickets.
The number of tickets that can be purchased, once defined, will be a mandatory requirement for the user when purchasing tickets.