The next number in the sequence is 24.
-7 + 7 = 0
0 + 7 = 7
7 + 5 = 12
12 + 5 = 17
17 + 3 = 20
20 + 3 = 23
Each two terms add the same odd number, and each pair adds the next odd number descending from high (7) to low. The first terms add 7, the second two add 5, and the next two add 3. The next odd number below 3 is 1, so add 1 to the last given term in the sequence:
23 + 1 = 24
24 is the next number in this sequence.
Then, 24 + 1 = 25… and so on…