Up to 40,000 customers could have had details stolen.
I find this number suspicious. Given the number of events Ticketmaster cover, the size of said events in terms of raw tickets, and the length of time, I’d expect the number to be substantially higher. And I’m assuming these customers don’t include the many football and rugby clubs that use tickmaster as their ticket engine, many of them can have 40,000+ tickets at one event (ok, so many customers will be buying more than one ticket, but unlikely to be in the 10s)