You'd need at least 3 Desktop Licences for those who develop dashboards and publish them to server, who will also need 3 server Licences.
All other consumers would need just server licenses, to interact with reports online. You can turn the web authoring feature ON so these users can connect to published data sources for ad-hoc analysis.
Get in touch with a Tableau Account Manager to get right direction on this.
Each desktop key will work in two machines. Three people going to develop the reports then buy two desktop user licenses.
Buy the tableau server license based on users view. Check with the tableau support and get license.