I use several kinds of clays in my soaps, and I can tell you in summary that:
- Smoother and longer lasting foam.
- Harder and longer lasting bar.
- Depending on the clay and its properties it may help the fragrance stay longer in the soap.
- The clays usually make the trace thicker or more viscous, difficult to make swirls or other designs with.
- It may become harder to cut if left a long time.
- They may affect the color of the final soap.
- The soap may become too drying on the skin, it may be helpful for people with oily skin (I guess that is a pro).
In conclusion, it is a matter of taste and as with everything else you must calibrate the quantity as to your satisfaction or the satisfaction of your customers.
But if you want a starting point, I would recommend that you start with 1% (of the oil phase).