If you are doing custom designs, you need a standard licence per customer, if customer B wants the same asset then customer A, you need to relicense. To use a sample design on your website, you need your own license. You can't post a design made for customer A without relicensing all assets used for that design. Standard licences are part of the subscription.
You only need to credit in editorial like situations.
Look here for more information on licensing: https://community.adobe.com/t5/stock/links-for-licensing-terms/td-p/11366788 (Disclaimer: As always with licensing, this is my interpretation of the rules. I think they are correct and advice is based on reading and interpreting the licence terms and on fair use for both the buyer and the artist/stock company, but I cannot rule out that my interpretation is wrong. I'm not an Adobe employee).
ABAMBO | Hard- and Software Engineer | Photographer