Confetti soaps are pretty forgiving. And they are a very good way to turn your mistakes into something wonderful. I have overcolored or overscented a couple of batches and grating those into an uncolored/unscented batch made some of my favorite soaps.
How much you use is up to your preference. Last night I grated 4 bars (4.5oz each) into a new 2KG batch. This makes for a pretty dense confetti which works well in this case because the confetti is large shredded and 2 intense shades of green against a base of plain white (used a little TD).
Others I have done were more subtle with a very thin shred mixed lightly into a similar color.
So play... have fun!