First of all, clouds form where air is rising. As the air rises, it cools because it is not only expanding, but it is traveling further away from the heat source. Now the air is saturated and the tiny particles are formed. These are droplets of water and are very very small, smaller than the head of a pin. Because they are so small they are easily kept up by the rising air. When enough droplets condense, they become visible and we see them as clouds. Not all rising air makes clouds, but if the air has enough moisture (humidity) in it, then clouds may form. So the reason that clouds can hold millions of water droplets is because the air in clouds is rising, and the rising air keeps pushing the water droplets up.

But cool air can't hold as much water vapor as warm air. So clouds that produce continuously falling rain or snow are thicker clouds, but usually lower (But not always). You might see these clouds as gray or blue-gray instead of the more common white ones. These clouds contain ice crystals and water droplets and may be a sign of a storm.