Sometimes job postings are for more than one vacancy/opening. It's useful to know whether a posting related to one opening or many. A simple property would address this:

totalJobOpenings
expected Domain: JobPosting; expected Range: Integer.
definition: The number of positions open for this job posting. Use a positive integer. Do not use if the number of positions is unclear or not known.

(Edit: this has been discussed in the Talent Marketplace Signaling W3C Community Group, see mail list archive for August thread on total job openings)