hmm thats odd, it should only take one, maybe you need to wait till it has taken it before you press again, the sound effect is a good sign that its done.
Its currently set to 1920x1080 but you can press F8 to get one at 8k, just be careful with that as a low end spec computer might not handle that.
You can always bypass the in built screenshot feature by pressing print screen on your keyboard and pasting that into an image editing app, also using shadowplay or OBS will let you make animations
Will be adding custom set resolution in the future