Are they only the Unix’s developed by Bell Labs or do the SCO ones count too? Are there others?
UNIX (POSIX) is a set of "guidelines" which must be followed by an operating system (and it's parts) for it to be considered a "UNIX".
So asking which is the UNIX OS that the others are based on or which was the first doesn't really mean anything. Sure there was an operating system which was the first to be coined a UNIX -- but really, what you're talking about it the guidelines it follows.
If your question is about code base lineage and not standards, current OSes directly descending from the original Bell Labs / AT&T Unix releases are AIX, HP-UX, Oracle Solaris & OpenSolaris derivatives all coming from System V release 4 and (Open/Free/Net)BSD and derivatives from the much earlier Unix version 6.
While Mac OS X has some BSD code, its core is not a descendant of the Unix kernel code.