Can i activate multiple activation keys
If you have an AWS tiered instance that you launched from the AWS Marketplace with (xx connected devices) in the title, it will be licensed and billed automatically through Amazon AWS. In such a case you would see "AWS paid AMI" on the Activation page of your Access Server and you can't activate another license on top of that.
If you have a subscription, monthly or yearly, then you can activate only one subscription on an Access Server. You can have multiple Access Servers on the same subscription, but you can't have multiple subscriptions on the same Access Server.
If you have a fixed license key, then you can activate multiple such fixed license keys on the same Access Server, and they will simply add up. Fixed license keys, recognizable for the fact that they are in the format of #EXA-MPLE-LICE-NSE#, can stack on a single server, but they cannot be used on different Access Servers at the same time.