toilet bowl sits between tub and vanity in very small bathroom.
On the vanity or get a free standing unit.
Sit on the toilet and see where it would be the easiest for you to take some off the roll and put it there. Convenience and accessability is your goal. No right or wrong spot.
Naomie hit the nail on the head.
We have the same set up in our tiny bathroom and that is just what we do. The TP usually sits on top the vanity or the edge of the tub and we have a free standing TP holder with multiple rolls on it a few feet away. You can also purchase a free standing single roll TP holder. If you decide you want to attach a holder to the side of the vanity, it can be done. You can find these at DIY stores in styles from basic to fancy to match your decor.
You can buy a stand alone piece that not only holds the tp but also holds the extra rolls. I use one in my bathroom. See here... https://amzn.to/2M0wrfm
Recess it into the side of the vanity cabinet.
Either purchase a ready made unit or diy.