Is it possible to have one reseller account for all my customers?

As a reseller you will have one joint reseller account. From this account, you can handle all of your customers (wholesale and retail) and all their domains from one centralized location. 

On your reseller account, you can add different domain groups for your various customers. Please see Reseller introduction: What are domain groups and how should I use them? or you can invite customers (retail model) to create their own accounts that will be connected to your reseller account and accessible via your reseller account.

How do I get a reseller account?

When you sign up to become a reseller, your existing Cookiebot account will automatically be converted to a reseller account. See How do I sign up to become a reseller?