I usually recommend budgeting one amp of 5vdc current for each servo attached. Might be a little liberal, but then what would you recommend?
That's probably a good rule of thumb for general hobby servos. Especially if you don't have any specs. However, some servos come with (or can be obtained) datasheets that list this information, so if you have that, use it.
This is especially important if you are trying to control 1/4 scale servos, or sail-winch servos which are much larger; those likely pull way more than an amp of current.
Finally - servos and DC motors don't generally require voltage regulation, you just need to know their current requirements so you can properly size the power supply.
