Calculation in Numbers to figure out the date 5 business days from another date
Days and dates are always messy, so whenever possible, one should try to use someone else's hard work. In this case, Apple have done it for us.

Say your date is in B2, example forumla to add five business days:
WORKDAY(B2,5,)

You can read more about its usage and find the other date-related functions available in Apple's Numbers Functions Reference.