Hi if I want to create AD accounts for contractors to work on site, would their accounts be still counted towards the CAL usage after they are gone and accounts have been disabled? If yes what are usually done to deal with this scenario?

A license agreement is a legal agreement between you and Microsoft.  This is a technical forum.  Your lawyers/auditors are not likely to accept the answers of a random reply from a technical forum as something that would stand up in a court of law.  Your best recourse on getting answers about licensing is to talk with a trained Microsoft licensing specialist.  You should be able to find one at the reseller from whom you purchased your licenses.  Or, you can contact Microsoft.  Start here - www.microsoft.com/licensing.