I've not seen this year's boot letter, but past years were very specific: You are instructed to buy a pair of the specified boots and low quarters.
Then, as Tug_Boat indicates, they will issue you a 2nd set of each on R-day. The are typically DLAT contract boots/shoes, and may not be a nice or comfortable.
So each cadet will start CBT with two pairs of each.
If you don't follow the boot letter instructions, or show up with out of spec or bad fit, then you will be issued (and charged) for an additional pair so that you start CBT with two of each.
If you show up with two pair of acceptable boots or quarters, they will not issue the 2nd pair. But you have to lug them on R-day, so that's not advised by most.
And if you picked wrong, hate your issue boots/shoes, whatever, you can buy what another CBT specified pair at the C-store, or have an identical pair sent during CBT. But most do not find it worth the trouble.
The reality is that you'll probably want "Inspection" shoes/boots anyway which are never worn and kept spotless. That's what happens to many of the issue DLAT boot/shoes, and that's OK. In DS's case, his issue boots were much cheaper construction, and they stay in his closet/locker (whatever). His low quarters were same as the Bates specified in the boot letter, and he does wear them for inspections, SOQ boards, etc. But others had opposite results, YMMV.