Incase you forgot the formula,
Number of BST's with 2 keys (1,2) = 2
Number of BST's with 3 keys (1,2,3) = 2 + 2 + 1 = (Root is 3) + (Root is 1) + (Root is 2) = 5
Number of BST's with 4 keys (1,2,3,4) = 5 + 5 + 2 + 2 = 14
Number of BST's with 5 keys (1,2,3,4,5) = 14 + 14 + 5 + 5 + 2 * 2 = 42
Number of BST's with 5 keys (1,2,3,4,5) = Root 5 + Root 1 + Root 4 + Root 2 + Root 3 (2 trees with 2 keys on either side)