The CRM licenses are priced on per user basis i.e., you will be charged based on the number of users you want to give access to CRM. However, you will also get same number of licenses for helpdesk, invoicing and other features.
CRM license is the core of everything, therefore even if you want to give access to a user for Helpdesk only, you will have to purchase license for CRM as well or otherwise they won't be able to see the customer details.