Regarding your inquiry on how to increase the number of devices that your Trend Micro subscription can cover, let me suggest for you to process the upgrade by clicking on this link. Enter the serial number for your current subscription then click on submit. The upgrade options will be listed under the Price List section and you may choose which upgrade to go with.
Note: Since you will be upgrading the subscription, you will be provided with a new product activation key to go with it. If you will be placing the same devices under this upgrade you may follow the procedure on the link below on how to change the serial number for your devices.