How many can you "own"? That depends on how many licenses you've purchased.
A single retail license of Windows 10 Pro provides you with one license. If you use it to install on your physical computer, that's it. You need to purchase another Windows 10 Pro license for each VM.
If you have Software Assurance or E3/E5 licenses, in certain cases you may be entitled to licenses for your VMs, ex: an "active Windows 10 Enterprise with Software Assurance or Windows 10 Enterprise E3/E5 subscription, which permits running up to four virtual machines concurrently."