Unlimited light agents can be added to Zendesk Professional and Enterprise plans. The cost is €25/license/month (minimum purchase equal to 10 licenses, so €250)
Why add Light Agents?
Get more of your employees involved in customer conversations. Light Agents can view tickets and add private comments, giving them visibility into specific issues and allowing them to collaborate with agents on solutions.
For a deeper dive, check out this
Using Zendesk as a light agent