How can I determine who takes up an agent seat in my account?
There are roles that require a Support seat depending on the product. For a complete list, the article: Managing agent seats for Support.
Suspended agents take up a seat. To free up the seat, Zendesk recommends downgrading agents first before suspending their accounts.
Light agents who are also Chat agents also take up an agent seat. For example, an account on Zendesk Suite has five agent seats. If the account has one admin, three agents, and ten light agents, we would think that they have one available agent seat. However, one of the ten light agents is also a Chat agent. This takes up the final agent seat, so the account cannot add any additional agents.
For more details on roles, see the article About team member product roles and access.