I generally go by the rule of thumb 1 gallon/day/calf. When the calves are little any more than that will make them scour; as they get older they can handle a lot more milk.
One of my nurse cows (#162) was milking ~3 gallons/day at the dairy before I brought her home. I put two calves on her - not three, mind you. My logic is that if by putting one less calf than I could on her, then if one calf gets more than his share the other(s) don't get shorted.
My other nurse cow (#311) milks ~6 gallons/day. I only have (had) four calves on her. She only has four quarters so I don't like to exceed that. :lol:
The other thing is that some cows let any of the calves nurse at any time. One or all, it doesn't matter. Other cows, it's an all-or-none deal. That's what 311 is like - which is another reason for not putting more than four calves on her. Someone wouldn't get to drink.
And yes...if she's not getting milked out that is bad news. I think you've got about a week after calving where it doesn't matter too much, but after that she'd need to be milked dry once they start nursing. If they drink half of what's in one quarter, a little out of another, and then one completely dry...that's asking for trouble. Usually you can get away with that once, but not on a regular basis.
That's why with young calves I like to pen them away from the cow, and then let them all together twice a day. That way I can make sure the cow gets completely milked out. Given a week or two I've been able to let cow and calves together and after that they run together 24/7. Some folks like to keep them penned separate -always- but that's a little too much work in my book. LOL. Put 'em out to pasture, grain the cow, and she'll care for them.