From either a plain cake mix or a wasc. How much sprinkles should I add? And does it matter what kind of sprinkles?

The packets that come with the mix are really small! I've added 3T and been happy with the results (the kids are happy I mean). I use what I call "jimmies"- kinda long and solid looking. Work great for me.

I make my own confetti cakes all the time (brides like to have their wedding colors).........anyhow I add about 1 Tblsp. per 3 c. of cake batter (a standard cake mix is like 5 or 6 cups I think)....so you would add 2 Tbsp.