I was a bit surprised to get the email, since I already bought 19000 earlier this year.
I was able to buy 3750 miles (but not 5000 miles) which surprised me even more.
So it looks like they have just upped the limit for everyone to 23750.
Technically we can purchase 4750 more - but I can't see any way of getting more than 3750, since the next increment takes you over the limit.