For most of the dives that you do when you complete adv Nitrox and Deco, your going to be doing about 1 gas switch. But as time goes on your going to expanding your limits. This is why they wanted me to carry two bottles, it was to get the idea of how to carry them safely and do the switches correctly. So when I practice doing it I'm going to do them correctly. I have tried to carry 3 bottles in the quarry.
On all of my non class deco dives, I do not carry more than I need too. This is why on some of my dives I carry a A80 with 50% and run a profile that might have me say at 20ft longer than if I were to switch gases to 100%. The question is. Do I want to have to carry / overload a dive that I can make simpler by staying at 20ft a couple min's longer.