The Quantity field is visible only if you use Commissions. Use it to specify how many commissions the employee should receive.
Other Payroll Items Area
These Other Payroll Items use the Quantity field:
Addition, deduction, or company contribution based on
quantity: Enter the quantity. For example, for a mileage
reimbursement, enter the number of miles.
Other tax based on hours or days worked: Enter the number
of hours or days worked.
The payroll item setup determines whether or not you can type in the
Quantity field. If you do not enter anything in the Quantity field, QuickBooks
calculates the amount for the payroll item without using the Quantity