I want to get the month of a certain date as a number...

The following code can be used to retrieve the month of a date in numeric format:

set theDate to the (current date) -- or any other date
copy theDate to theFirstMonthDate
set the month of theFirstMonthDate to January
set monthNum to (1 + (theDate - theFirstMonthDate + 1314864) div 2629728)

This feature was introduced nativelly in AppleScript 1.9.2, so you can now simply ask for “January as number” and you will get the integer “1”.