The Attendees page shows 20 attendees at a time. Ticking the checkbox at the top of the list selects only the 20 attendees on the current page, not all of them. To resend tickets to every attendee in one action (no batches), use the Select all link that appears after you tick that checkbox.
Select all attendees and resend their tickets
Go to your event, then click Attendees.
Tick the checkbox at the top of the list, next to "Showing 20 attendees". This selects the 20 attendees on the current page.
A second option appears: "Select all 20+ attendees in your store". Click it to select every attendee in the list, not just this page.
Check the selected count matches the number of attendees you expect before you continue.
Click Actions, then choose (Re)-Send ticket emails.
Ticket emails are sent to all selected attendees at once. You don't need to resend in batches of 20.
Only want to resend to attendees who didn't get a ticket?
Filter the list first: on the Attendees page, click More filters → Ticket delivery status → Not Sent. Then follow the steps above. The Select all link selects every attendee that matches your filter.