CO tends to disperse quite evenly through a building so you really needn't have more than one per floor and/or group of bedrooms. The biggest concern is to avoid dead zones where air stagnates. Keep CO and, smoke detectors for that matter, at least 12" away from inside corners, doors and away from areas likely to be covered by drapes, furniture and from behind doors that might block air flow.

Around here CO detectors get mounted about knee height and typically in the hall and outside bedrooms as this keeps them out of the way and out of sight lines. It also limits the tendency of people to put furniture in front of them. But as long as you keep a decent distance away from dead zones height is not critical.

CO poisoning is usually a slower process compared to fire and, because of its tendency to mix and spread, usually large areas of the house are effected. Which means that unless there is a potential source of CO within a bedroom, perhaps a small gas heater, then it really doesn't make sense to have one in each bedroom.

As long there is good coverage more CO detectors are not necessarily better. Not if buying more means you buy cheaper ones. And not if after ten years or so, when the CO and smoke detectors need replacement, you put off replacing them.