In general, no.
We mostly plate our disks as they are ordered. The main reason behind this is that many people require a center hole in their disks. These have to be cut in the blank disk before it is plated.
We do have a few more popular sizes and grits that we try to have on hand from time to time, but it is very limited.
In most circumstances the longest turnaround time is about 4 weeks for a disk. It would follow this scenario:
We just finished plating 24 inch disks. You call and place an order for a 24 inch disk. We have to plate through our 12, 14, 16, 18, and 20 inch disk orders to get back around to the 24 inch disks again. On average, this takes about 3 weeks to do. Then we plate your disk with the other 24 inch orders and it goes out the door.
We maintain a rotating schedule in order to cut costs and improve efficiency. If you happen to order right before we are plating the size disk you need, it will ship out very quickly. It's just random.