Yes, you need to have a separate license for each website. You might need to purchase extended license for your web application.

Regular license can be used for end products that do not charge users for access or service(access is free and there will be no monthly subscription fee). Single regular license can be used for single end product and end product can be used by you or your client. If you want to sell end product to multiple clients then you will need to purchase separate license for each client. The same rule applies if you want to use the same end product on multiple domains(unique setup). For more info on regular license you can check official description.

Extended license can be used for end products(web service or SAAS) that charges users for access or service(e.g: monthly subscription fee). Single extended license can be used for single end product and end product can be used by you or your client. If you want to sell end product to multiple clients then you will need to purchase separate extended license for each client. The same rule applies if you want to use the same end product on multiple domains(unique setup). For more info on extended licenses you can check official description.

If you are charging your customer for using your SASS based application you must buy an Extended License for each end product. If you aren't charging your customer then purchase Regular License for each end product.