No, I didn't mean "hard" limit, as in "Tg won't allow more". I mean 3 layers is a practical limit, for cloud layers of reasonable depth and density. If you have three layers, all casting shadow on the layer below, there will be almost no lighting left for the lowest, except GI. But of course that depends on depth and density too.
You could use a trick, Richard: don't make the large cumulus so big. Instead, make them like 500 to 1000 tall, but pull them much lower, so that they appear bigger.