You could use number of available CPUs as your pool size.
Result:
CPUs 8
thread with value 1 current sum 3
thread with value 2 current sum 3
thread with value 3 current sum 6
thread with value 4 current sum 10
thread with value 5 current sum 15
thread with value 6 current sum 21
thread with value 7 current sum 28
thread with value 8 current sum 36
final sum 120
thread with value 11 current sum 66
thread with value 13 current sum 91
thread with value 15 current sum 120
thread with value 9 current sum 45
thread with value 14 current sum 105
thread with value 12 current sum 78
thread with value 10 current sum 55