Puzzle ZBOM 

Workings

Hint
Look at the first letters of the months.

Answer
199.

The first part is the alphabetic position of the first letter of the month, S=19, the second part is the number of letters in the word itself, September=9.
January    10-7
February   6-8
March      13-5
April      1-5
May        13-3
June       10-4
July       10-4
August     1-6
September  19-9