Shortages happen every year for every apple product.
We'll have to see of course. This is going to be a 3rd option (by rumors) with edge to edge display size of the Plus but nearly in the normal iPhone size - while also bringing out the regular 7s and 7s Plus (even if they call them 8's). So they'll undoubtedly charge more for this OLED option....maybe with the price high enough, demand will be tamped down and all the peons (myself included) will get regular (non edge to edge OLED) 7s's.
Since this (edge to edge iPhone display) was broadcast for the last several years Samsung is pushing to have this for their s8 in March and since all their production is already OLED they could do it for their whole s8 offering (making it difficult for Apple in Sept. to push this on the market as only a higher end offering). Time will tell.