Singular:
You have selected %1$s attendee for %2$s. You can move it to a different ticket within the same event, or to a different event. You have selected %1$s attendee for %2$s. You can move it to a different ticket within the same event, or to a different event.

Plural:
You have selected %1$s attendees for %2$s. You can move them to a different ticket within the same event, or to a different event. You have selected %1$s attendees for %2$s. You can move them to a different ticket within the same event, or to a different event.

This plural form is used for numbers like: 1

This plural form is used for numbers like: 2, 3, 4

This plural form is used for numbers like: 0, 5, 6