Question

What is the impact of plastic money on money demand and supply ?

Answer 1

I assume you mean credit cards and debit cards when you say plastic money. In both cases, the real money is in the bank. If you use a debit card at a store, you are transferring your real money in your checking account to the store's checking account. If you use a credit card to buy from a store, you are borrowing real money from the bank, and it is transferred to the store's checking account.

Debit cards have no effect on the money supply.

If credit card users charge more purchases, I think it would increase the money supply, just as if people borrowed and spent more.

If you are asking about the "supply and demand" for money, I don't know how to answer that.