It is expected that iPhone 8‘s launch is going to be in September; but, according to reports, the phone’s availability will be extremely limited in quantity and most potential users will not be able to buy it that month.
There have been researches on the iPhone‘s availability by Barclays’ analysts Blayne Curtis Christopher Hemmelgarn, Thomas O’Malley, and Jerry Zhang (source) that the stock in the months of its release which is September will not fulfill customer demand and most of the customers will have to wait for the second batch to get their hands on the device.
Suppliers generally had good things to say about the upcoming iPhone 8 launch (for our purposes iPhone 7s, iPhone 7s Plus, and iPhone Pro) as new features drive a more complicated manufacturing process and higher ASPs. We now believe that all three devices will feature wireless charging and will all be launched in the normal September timeframe, although the majority of iPhone Pro volumes may not be available until Q4
You could still pre-order the device whenever that is available but you must be extremely quick with the order, or have to wait for the next shipment of the iPhone 8 when it arrives.