If the end users need to pay to see the end product, you need an Extended License. There can be more than one end user as long as there is only one end product.

If the item is part of a product for sale, you need the Extended License.

As long as all end users can freely access the end product, the Regular License is fine. There can be more than one end-user (the person who uses the end product, such as the end consumer) as long as there is only one, unique end product.

No, just using the item in a commercial setting doesn't necessarily mean you need an Extended License. You need an Extended License if the end product is sold to end users. If the end product is free, even if you are a commercial enterprise, you only need a Regular License.

No, the Regular License is fine. You only need the Extended License if the item is actually part of the product itself.

If the item is used within the free part, the Regular License if fine. If the item is used in a part that requires payment to access or make use of, you need the Extended License.

Sometimes an author will choose to distribute a theme or plugin item on the basis that it's 100% GPL (GPL stands for "general public license" and is an open-source license). If so, this is noted on the item page and license information will be included in the download files. In these cases, the GPL license applies to the whole item instead of the terms of the Regular or Extended License. Different versions of the GPL exist and the relevant version of the GPL will identified. You will need to make sure that you read and abide by the GPL terms. Other items have a split license - more info on that here

Please see this KnowledgeBase article on how split licensing works.