Don't Expect an iPhone-like Sell Out
It can be difficult to grab the iPhone model you want during the pre-order period. The iPhone often sells out within hours of going on sale with shipping times pushed into October. We don't expect this to happen to the iPad Pro.
If the iPad Pro goes on sale at the same time as the iPhones we could see pre-order issues emerge as sites struggle with the load, but we'd expect most iPad models to remain in stock as people fight to pre-order a new iPhone.
We haven't seen any rumors point to shortages or delays at launch so we expect there to be plenty of stock available from the start.
We could see shipping times on the more popular color/storage variants get pushed, but don't expect a complete sell out.
That said, if you want a very specific color/storage combination, you might want to put in an early order just to be safe.