I'd argue that TOS was more internally consistent than later series - but out of practical reasons that there were fewer episodes to mess it up.
Retconning TOS is easier than retconning other shows, yes, thanks to there being less of the (always inherently faulty) material to mend. And also because generations of fanboys have spent time and effort doing so already, and have sometimes worked their favorite solutions back into the Trek universe, by directly contributing to the shows and movies, or by influencing people working on those.
OTOH, TOS is more inconsistent inherently than the others, because of not being conceived as serialized. OTTH, it's more static than the others, as there was less impetus to introduce changes than in a show expected to stay on air for the better part of a decade come hell or high water. OTFH, it still lived long enough to go through a number of people in charge of pseudotechnology and aesthetics, while several of the seven-year spinoffs enjoyed stability in this respect.
In the end, though, a count of hands isn't all that relevant. It's just Star Trek, part of a larger whole that exhibits insanely good continuity for an entity that wouldn't really need to. Indeed, the continuity is a prime attraction, no doubt contributing to its considerable recognizability over perhaps equally long-lived but more anthology-type fictional universes.