One of the nuggets of wisdom my mother bestowed upon me (along with “necklace or earrings…never both” and “NEVER trust a man in white trousers”) was “display things in groups of three”. I find this works particularly well when hanging pictures or photographs. A pair can look lost, but four can look cluttered. 3 really is the magic number! Trios of decorative accessories have maximum impact and allows you to show varying sizes and heights of the same item.
Grouping interesting items in multiples, in odd numbers rather than even, makes the item look like a collection. Whether said collection is clustered or spaced apart equally, it gives a personal or thematic touch to your room.