My 32GB Samsung Galaxy S7 has only 24GB of storage space when I opened the box. It shows 8GB ate by the Android operating system, I think the TouchWiz also included in that OS thing. I have updated my OS, updated with contacts, apps and email settings. Now my Galaxy S7 shows only 15.5GB of free space without taking a single pic from my camera, nor shooting any videos.
I have made a mistake, I would have bought a 64GB version of Galaxy S7 to get more space. But, one relieving factor is, I can add a microSD card and move the installed apps to the SD Card that would further saves my internal memory by 3-4GB for sure.