DON’T SAY: That grocer has plenty of clients.
SAY: That grocer has plenty of customers.
DON’T SAY: That lawyer has plenty of customers.
SAY: That lawyer has plenty of clients.
RULE: A person can be a CUSTOMER at a shop, but a CLIENT of a lawyer or a bank, where they purchase a professional service (industries such as advertising, real estate, business consulting, legal services, financial planning, etc.)