If you have a Creative Cloud for Teams license you can add new licenses. Please see this document. Manage your teams account
If you have Creative Cloud for Enterprise your licenses will reflect what is in your agreement. You can overdeploy any licenses and account for this during the annual true-up.
Please note that if you have purchased an 'All Apps' plan for Creative Cloud this will include Illustrator.
If you are an individual user and wish to purchase Illustrator see Buy Adobe Illustrator CC | Download graphic design software free trial