The ISE ordering guide has a very good overview of the licensing requirements.
https://www.cisco.com/c/en/us/products/collateral/security/identity-services-engine/guide-c07-656177.html
Each virtual machine ISE node you deploy requires a specific VM licenses. These comes in three sizes to support the various ISE templates sizes. These ISE templates you use for virtual ISE nodes match up to the physical SNS appliances you can order. All of these nodes sizes have different stated scaling capabilities, but they aren't throttled or slowed down in any way. Endpoint licenses, base, apex, and plus, are all shared across any node you add to the deployment in a common license pool.
Small VM license - SNS3515(16 GB) and SNS3615(32 GB)
Medium VM licenses - SNS3595 (64GB) and SNS3655 (96 GB)
Large VM licenses - SNS3695(256GB)
This link shows the SNS platform templates that the VM's are modeled after.
https://community.cisco.com/t5/security-documents/ise-performance-amp-scale/ta-p/3642148#toc-hId-1174996758
