I would prefer Select and Filter actions over Apply to teach and Condition.

DayDiff: Compose Action with expression:
div(sub(ticks(outputs('EndDate')), ticks(outputs('StartDate'))), 864000000000)
DayArray: Select action:
From:
range(1,outputs('DayDiff'))
Map:
addDays(outputs('StartDate'), item())
WorkDays: Filter action
From:
body('DayArray')
Filter:
@And(greater(dayOfWeek(item()), 0), less(dayOfWeek(item()), 6))
CountWorkDays: Compose action
length(body('WorkDays'))
This counts from StartDate+1 up to and including EndDate.