There's an assumption here that a cloud is a static thing that fills up and then empties. This isn't the case. A cloud is merely a manifestation of the moisture in the air. The water condenses slightly to form a vapour. It's part of the water cycle. So as it's raining it could also be filling up from evaporation, etc.
I suppose the short answer is the (maximum) amount of rain in a cloud is dependant on the air temperature, relative humidity goes up as the air increases in temperature. Here's a graph:
So to pick a slightly arbitary figure at 60f(15c) every m3 of water in the air (can) hold just under a pound of water (I don't know why they choose pounds and meters here, meters and KG would be a better option but there you go).
Obviously cloud temperature can very considerably and clouds can be many km2 in size. Clouds can reach a height of 20,000 feet and come in lot's of forms:
If you look at the cumulonimbus cloud (a common "rain type" cloud) the temperature gradient from the top and the bottom (50,000 feet in total!) can be very large indeed as is the m3 of moisture.
If we take our cumulonimbus and assume it is 50,000 feet (15240 m) high. Let's assume the average temperature of this cloud is (picks a number at random) 33f(1c) that means every m3 holds a bout 0.3lbs of water. So a 1x m block 15240m high can hold about 4,572lbs of water. You then need to times this by the area (m2) of the cloud (if you can figure this out...).
Obviously we make a lot of assumptions here and it will not be a static amount (pretty much ever). Also bear in mind that not all this moisture will turn to rain.
So tl;dr a lot and it's complicated.