Create a microflow with a variable startingDay and a counter. Check for each day if it is a saturday or sunday by using formatDateTime(startingDay, ‘E’). The E is for Day name in week. You also could use u for Day number of week. If it is a saterday or sunday do not a 1 to the counter. Then use the addDays function to add a day and keep doing this untill you are at the enddate. The counter will tell you how many working days you had.
Have a look at the WorkdaysBetween java action in the ApprontoCommon module in the marketplace:
I have never used it myself, but it seems to do just what you want.
It must be simple with a custom java action.