Today I found an information from Zendesk that should cause the confusion and/or mis-understanding:

Per this subscription, I should be able to set up 2 Agents within my account. However, after creating 1 Agent, Zendesk said that there was no slot. At this moment, I realized that Zendesk has considered the Admin account as an Agent.

This caused a problem for our company since we have 2 Customer Supporters. For me, as an Admin, definitely I have all the powers but I will never interact with Customers, that's the job of Agents.

Therefore, it will be better if Zendesk put a disclaimer that: 2 Agents will be 1 Agent + 1 Admin, not "2 Agents are 2 Agents".

A bit disappointed about this.