Entitlement Confusion

Hi,
I have 1 active subscription, activated with a live server running.

I need to buid out a test/dev server, I notice that Im using 50% of my entitlement, does that mean I can install & activate another server on that subscription, or do i need a second subscription for my second server?

Tried to hunt for a answer, but all FAQs were as clear as mud.

Thanks