The amount of ventilation depends on how big the coop is and how many chickens you have. In general, you should have the ventilation up high and you should have ventilation on the north and south sides of the coop. Make the ventilation openings adjustable so you can close the north side ventilation in the winter (to prevent drafts) and you can open them wider if your coop starts to smell bad or get really warm/humid.
For winter you want vents up high on the walls (preferably protected by a roof overhang), each of them being closeable so that you can adjust how much and which direction is open depending on your weather.
For the warmer part of the year you will want additional ventilation, which could be big windows (with predatorproof screens) and/or parts of the wall that have removeable panels (to predatorproof screens) or something like that.
There's no such thing as too much ventilation, assuming you have a way of covering the different parts up to manage how mcuh is open on a given day. And it is WAY easier to build in lotsa ventilation in the construction stage than to have to hack holes in a completed coop
I don't think it's possible to give a really accurate number for how much ventilation you will need -- it depends so much on climate, coop management style, etc etc -- but I would not suggest building in any less than 1 sq ft per chicken, and in some situations you will strongly benefit from more than that. Don't do little holes, they are basically useless, have actual decent-sized openings (with hardwarecloth to keep predators out) with flaps or sliders or whatever to allow you to close them off when you want to.
I have a page on the subject that might also be a little useful to you, see the link in my .sig below