If you search "openvpn" within the AWSMarketplace you will receive a number of options. Differences between numbers of users is self-explanatory, however there is a difference between the first and second option that is shown on this results page.
Both of these are pre-built Access Server images that are ready to deploy.
The first option is a Bring Your Own License image - in order to add users and allow them to connect, an Access Server license will need to be purchased and activated on the instance. Licenses can be obtained from our website https://openvpn.net/index.php/access-server/license-key.html?paynow=SALE
The second option (and all of our other AMIs) come with licenses attached and are ready for your users to connect as soon as they are deployed.
What are the differences?
AMIs with licenses attached -
Both image types will incur standard AWS usage charges based on the size of the instance.