If you've been through Apple's iPhone pre-order process you know it can be hard to snag the model you want. Most iPhone models sell out within hours of going on sale and shipping dates are often pushed into October. We could see some shipping delays impact the Apple Watch 4, but we'd be surprised if every model sells out immediately.
We haven't seen any rumors point to shortages or delays at launch so there's a very good chance Apple and its retail partners have enough supply to meet the initial demand.
Some of the more popular color/storage variants could see their delivery dates get pushed, but don't expect a complete sell out.
That said, if you want a very specific color/storage combination, and you want it ASAP, you'll probably want to put in a pre-order just to be safe.