in Splunk documentation, there isn't a clear requirement for Deployment Server infrastructure, they generally says that it's the same of other Splunk systems (at least 12 CPUs and 12 GB of RAM).
If you use the Monitoring Console Health Check, it says that at least you need 12 CPUs and 12 GB of RAM.
My hint is: usually Deployment Server is a Virtual System so start with a less configuration (e.g. 8 CPUs and 8 GB of RAM) and then monitor performances with the Monitoring Console and eventually improve configuration.
The only clear requirement is that, if you have more than 50 deployment clients, you have to use a dedicated server without any additional role!
If it's a physical server it's better but there's no problems with a virtual server.