The 32GB Samsung Galaxy S6 & S6 edge have 23GB of free space, so consider going 64GB
Then again, it's not the Samsung Galaxy S6 at fault. As smartphones evolve, storage space stays exactly the same, which means another new device, such as the HTC One M9, too comes with about 23GB of usable storage out of the box.
The key takeaway here is to think ahead about your memory requirements, and save up if needed. To aid you in the decision, here's an approximation of how much space you're getting with the different versions of the Galaxy S6 and S6 edge - you'll get around 55GB free on the 64GB model, and about 119GB free on the 128GB one. Although none of them have SD card slots, cloud storage is a good option for storing volumes of many files of smaller size - such as your photos. But how good of an option it is is also dependant on the quality of your internet connection.
source: Android Central