If multiple effects modify your hand size, apply them in timestamp order. For example, if you put Null Profusion (an enchantment that says your maximum hand size is two) onto the battlefield and then roll while Undercity Reaches is the face-up plane card, you'll have no maximum hand size. However, if those events had happened in the opposite order, your maximum hand size would be two for as long as Null Profusion is on the battlefield.