Many companies or organizations use the term FTE or Full time Equivalent. If two people split the 40 hours a week, they would make one FTE. I would specify the interns as another category.
It would be acceptable either say we have 4 Full-time and 6 part-time employees; or we have 8 FTEs.
The contractors should be noted separately, though they can also be stated by FTE.
For the purposes of an unofficial survey I would go with the FTE # for he actual employees, and give 1/2 credit for the interns. I would ignore the contractors in the count.
If the survey wanted to know how many people worked in the office space, and didn't care how they were paid, I would add the contractors to the count. The landlord might want to estimate the number of parking spaces you will need.