My water bottles are 21 oz and I use one tablet. But... I have put a second tablet in when the bottle is only halfway gone, especially when there's ice in the bottle.
I don't get sick from any of it and it feels much lighter than anything else I've used, like Accelerade or Gatorade. I haven't used anything but Nuun this season, even on 50 mile rides. I just use Shot Blocks and real food.
I want to try the Skratch Lab stuff, but I have a lot of Nuun to use up.