It used to be pretty slim, as carrier stores used to be pretty busy. Last year, however, there was a bit more stock than usual because they launched 2 different phones at two different times. With the XS and XS Max being normal S upgrades, mixed with the fact that a lot of people didn't get their X's until Nov/Dec/Jan, I don't think there will be a big rush this year. I personally anticipate more than normal store stock, but also take into account where you live. May not have as much stock in smaller towns, but also not as many people going to their stores. Stores won't tell you their stock numbers in most cases, and even if they did it wouldn't be until Friday.
There will be plenty to go around in carrier stores and apple store. If your store doesn't have the model you want on Friday, they'll have it Monday. Just call and find out. This is a down year, XR is what most are waiting for. Apple wont be able to give these XS phones away (for the absurd price) by the holidays.