Ideally I think you would want to have at least one iPhone and one ipad since they are two different aspect ratios but either way you will want to get other people to test it on other iOS devices. I used testflight and got some people to test it out that way. For the screenshots I just used the ipad screenshots and then cropped and resized for all the different required resolutions which you may or may not be able to do that depending on your game and gui elements.