"Helpdesk agents" (AKA "technicians" or "techs") is a common term in help desk software used to describe a person, who "handles" tickets. By "handling” we mean responding to customers, closing tickets, changing ticket details, moving tickets from one category to another, adding tags, etc. Simply put, an agent is the person works with customer problems.
Now, we don't charge per "helpdesk agent", but the number of agents you can have is limited by your current Helpdesk plan. To clarify let's say your plan includes up to four agents, it means that you can have four people handling customers inquiries, viewing reports, managing knowledge base and so on.
Agents are not to be confused with regular users. We do not limit the number of users or the number of tickets those users can create. You can have a million customers, each creating five tickets per day, it doesn't matter. What is limited is the number of people who respond to customer requests.
In Jitbit Helpdesk we refer to agents using the term "technician” or "tech”. A Technician is a user with permissions to a particular category. This approach lets you have groups of technicians working with their tickets completely separately from each other. For example you can have sales and IT departments working together in a single helpdesk app and they will not see tickets in each other's categories, unless you want them to.