It's best to use a singular subdomain vs. multiple subdomains. I'd recommend something like the following:
staging-admin.domain.com or staging-cms.domain.com or stg-site-admin.domain.com
staging.domain.com or stg-site.domain.com
You can generate all those license keys without issue in the Client portal for free, you may just have to wait a day or so for some of them.
I'd highly recommend against using something like:
admin.staging.domain.com. Many folks we've worked with have internal network issues that don't allow or play well with multiple subdomains.