Each Atlassian license can be associated with an unlimited number of 'Technical' and 'Billing' contacts.
There is always one 'main' Technical contact and one 'main' Billing contact -- these are the people we contact for specific quesitons about the license. However, you can also add more technical and billing contacts.
Technical contacts have the ability to request Support for their license.
Billing contacts are contacted regarding payment of licenses and invoices.
If you're not a big company, it's perfectly OK to have one person listed as both the Technical contact and the Billing contact.
To add or remove Technical and Billing contacts, go to https://my.atlassian.com and login as a user who is already associated to a license. You can then add and remove other contacts.
If somebody in your company leaves, or somebody joins your technical team, be sure to add them as contacts so they can get Technical Support and be informed of product-related information such as new version releases.