No, not one address for each client, one for each transaction. A single client who makes several different purchases will (normally) be given a separate address for each one. It's not difficult to manage at all. Most merchants will already have a database to keep track of individual orders; the bitcoin address that goes with each order is just another entry in that database, and it's easy to automatically monitor these addresses for incoming payments (and a payment processor such as BitPay will do the latter automatically).