Touching Dry Impurity
It’s a matter of personal hygiene and prevention from spreading of infectious diseases and eating or drinking something which you cannot even think of in your normal life. Washing must be done till you are satisfied that your pots, in this case, physically looks clean. I have not come across any numbers to which you must clean. Yes in Hindus its 7 times, as far as I know. We must have borrowed it from them in our culture. This is my understanding and I’m not a religious scholar. Hope this answers your question.