Hi JGS and welcome to the Samsung Community!
This depends on your location and provider should you purchase one with them. This is because a certain amount of space is reserved for the Operating System and another part is reserved for Samsung and provider partner apps. Which apps are pre-installed depends on the country and provider.
In the Netherlands (as you've reached the Dutch community), there are to my knowledge no provider pre-installed apps anymore, so all you're dealing with is the Operating System and some partner apps which can be disabled leaving just a small installation file behind. All with all, based on the S7 running the same Android version, I expect that to be 5-7GB.
Also, since you've reached the Dutch Community, should you happen to speak Dutch, please do 
Have you yet decided on the Galaxy S8? Should you have any other questions, don't hesitate to ask.
