Yes, it is possible for you to customize all parameters related to your subscription with full flexibility and based on your needs, you can also change those at any time.
Customizable parameters and quotas include:
Number of Users
Seller Central Accounts
You can change your subscription details in the subscription page of your account, by clicking on the dropdown related to each parameter and picking your desired number. Your custom subscription price can be seen on the right side as you change your subscription parameters, it gives you a precise idea of the breakdown of your subscription cost.