There might be a clever VBA Function You can use in an arithmetic expression in Workflows.
A simple, purely imperative solution could be:
Create a loop in a workflow, where you iterate from your "from"-Date to your "to"-Date in DateInterval.Day intervals with the DateAdd Function.
In every iteration check, if your current date is a Workday with the Weekday-Function. Increment a counter variable if it a weekday, otherwise don't.
Beware that there is a safeguard in workflows, that prevents exceeding the number of executions of a workflow activity within a certain amount of time (I believe the threshold is 50?). You shoud find KBA-posts on that. If you have to deal with large numbers of loop iterations, have the workflow wait after a certain number of loops.