Regular License is fine when end users (the end users here are your client’s users, not your client himself) can use/view the end product (the website) for free. And Extended License is required when end users will have to pay to use/view the end product.
So as long as your customer is planning to publish his website for free, a Regular License is fine. Otherwise, an Extended License is required.
You cannot use an Extended or a Regular License on multiple clients, you need a license for each client.
I hope this help.