I will explain you my way for keeping arrows in hand. Of course, this is not the only one but this one works well for me 🙂
3 arrows :
Clasped by the whole hand against the plate of the handle of the bow.
The thumb holds the arrows and the cock feathers look at the bow.
6 arrows :
Taken by the middle finger, the ring finger and the little finger. Held against the thumb.
Cock feathers look at the window of the bow.
10 arrows :
4 arrows more holded by the forefinger. Cock feathers look at the window of the bow.
13 arrows :
3 arrows more holded by the little finger. Cock feathers look at the bow.