I have to reinstall McAfee at an office. The people at the office say they have already bought it.
Will there be a licence number that they can give me to type in when I reinstall it?
This question is unlikely to help any future visitors; it is only relevant to a small geographic area, a specific moment in time, or an extraordinarily narrow situation that is not generally applicable to the worldwide audience of the internet. For help making this question more broadly applicable, see the FAQ.
If "the people" bought legitimate licenses of McAfee they will have a license key (or license keys) or a licensing server/service that will track the used licenses on the network. It is more likely they'll have license keys due to assumptions I'm making regarding your Information Technology setup (if you're the IT person and you don't know about their setup, they probably aren't running a licensing server).
Whoever made the purchase should have records regarding when the purchase was made and what sort of things came with the purchase. Usually, any license keys will be included in documentation that is sent from McAfee to you after the purchase.
If you don't have a license key and nobody can find one, use the proof of purchase documentation, Purchase Orders, receipts, etc, and contact McAfee support to find out about getting a license key.