With 13 different operands you can make 4096 different sums. By grouping into 2s, 3s, etc, using parentheses you can increase this by 77=4173. Then you can use irregular grouping: one pair and the rest of the operands ungrouped (12): two pairs and the rest ungrouped, and so on. Similarly, starting with a group of 3 (trio), and the rest ungrouped; then mixing trios and pairs; and so on.