The need of rotation will vary upon application and circumstance. So ideally infinite... so much as I can program the motor to have travel stops.
I thought it was for "a personal passion project", but now it sounds more like you want engineering done for a large commercial range?
Precision is a must, the most accurate, repeatable, and small increments is desirable.
What part of "rigid quantifications" didn't you get?
Terms like "the most accurate" and "small increments" are not metric quanta (unless we're talking Fuzzy Logic). In your case, degrees should be a useable nominator and I'd be surprised if you need something much more precise than what you can do by hand, but you are the one having to spec out what's needed and you have to do it in terms that means the same to you as to everyone else.
And... Never go for a precision which is far beyond your real needs, as it will at least make it more expensive and harder to do and Worst Case, it might result in no answers at all.
Repeatable action is also a must with accuracy... for what is accuracy if repeating the action is inaccurate?
What is accuracy if not quantified?
Accuracy could be eg. +/-2° with a repeatability of +/-1° and that would be engineerable, superlatives, on the other hand, doesn't really cut it until after the job's done
When you go shopping, do you eg. say "I'd like several bottles of milk of a rather lean quality please" or do you say something like "5 bottles of skimmed milk (and a single bottle of Jack Daniels to cure the milk breath) please"?
The motor does not have to be strong. It is simply rotating the barrel of a lens (can you guess what I am building yet?).
So, does the barrel have any mass? Does the thread have friction? Does it have to do the job within a certain time frame? Or will even the weakest motor imaginable do?
I understand details are important,
so forgive me for being ambiguous.
Don't confuse confusion with ambition.
I hope this helped!
It didn't, try pushing some numbers.
Stepper motors are more precise then servo... right?
Well, it depends... If you're going many revolutions they can be, but within eg. a single revolution, their precision depends on how precise the poles are mounted. If gear reduction is used, precision suffers.
In a servo system, the motor type doesn't matter for the precision, as it is all in the position feedback system.
If you're solely talking about R/C-servos (those used for remote controlled toy cars) and you want several revolutions, the position feedback system has to be removed, so it's not really a servo anymore, but rather a tiny geared down motor with a motor controller reacting to pulse width signals following the R/C protocol.
Since you're not really keen on quantifying your needs, they're impossible to provide an answer for, but did you consider how you're gonna use it, once it's working - perhaps that would help you define your needs and put them into numbers.
Until such time that numbers start entering the specification in a reasonable amount, please don't expect anybody to have a chance of helping you out here.
Engineers aren't oracles - we just seem to be since we make the world go 'round