I have worked on a flow that will calculate the end date given the start date and the number of working days. This formula will exclude weekends. You might be able to use this to get the working days. Another option would be to use an unattended UI Flow and run a calculation in Excel to return the number of working days.
https://docs.microsoft.com/en-us/power-automate/ui-flows/overview
This is the expression in Compose 2 - Compose 3 outputs the end date.
if(equals(dayOfWeek(adddays(if(equals(dayOfWeek(variables('startdate')),6),addDays(variables('startdate'),2), if(equals(dayOfWeek(variables('startdate')),0), adddays(variables('startdate'),1),variables('startdate'))), add(mul(7,div(variables('duration'),5)), mod(variables('duration'),5)))),6), addDays(adddays(if(equals(dayOfWeek(variables('startdate')),6), addDays(variables('startdate'),2), if(equals(dayOfWeek(variables('startdate')),0), adddays(variables('startdate'),1),variables('startdate'))), add(mul(7,div(variables('duration'),5)), mod(variables('duration'),5))),2), if(equals(dayOfWeek(adddays(if(equals(dayOfWeek(variables('startdate')),6), addDays(variables('startdate'),2), if(equals(dayOfWeek(variables('startdate')),0), adddays(variables('startdate'),1),variables('startdate'))), add(mul(7,div(variables('duration'),5)), mod(variables('duration'),5)))),0), adddays(adddays(if(equals(dayOfWeek(variables('startdate')),6), addDays(variables('startdate'),2), if(equals(dayOfWeek(variables('startdate')),0), adddays(variables('startdate'),1),variables('startdate'))), add(mul(7,div(variables('duration'),5)), mod(variables('duration'),5))),1), adddays(if(equals(dayOfWeek(variables('startdate')),6), addDays(variables('startdate'),2), if(equals(dayOfWeek(variables('startdate')),0), adddays(variables('startdate'),1),variables('startdate'))), add(mul(7,div(variables('duration'),5)), mod(variables('duration'),5)))))
