As an agency should I create one Storrito account for each of my customers

Short answer: yes

Storrito is designed for single-users and small teams. If you are an agency we recommend you to create one Storrito account for each of your larger customers. Otherwise the Storrito gallery and other parts of the app will get to crowded.

As a Storrito user you can be part of multiple teams (Storrito accounts). The article below explains how our Storrito team feature works:

https://help.storrito.com/articles/2483377-team-feature