Pay periods for seasonal employee

[GSBrandt]

We have an employee who is salaried and works (and is paid) for 9 months of the year. The employee is paid twice a month, and we divide her salary by 18 pay periods.
How many pay periods do we select for this employee to ensure the tax withholding amounts are done properly?

[Paul Mayer]

I would use Semimonthly as the Pay Period.