First time I didn't wash it off after I applied it, and my partner went numb as well. I've found the best way for me is to apply the product well before any action, let it dry and take effect, then thoroughly wash the area.
How much? Depends on what kind of effect you want. Too much and you can lose pretty much all feeling, too little and you won't think it works at all. You have to find your own balance, I suggest starting out with more than you think, then you can scale back if you need for the next use.