Do I have to buy a Confluence license if I want to use the knowledge base in my Service Desk or is there another way to do that? I want to be able to have some documentation on basic requests so the customers would have access to it.
You need to pay for Confluence license, though it doesn't necessarily need to be big, as the unlicensed Service Desk users don't count towards the total license count in Confluence.
The smallest deployment is just enough for the JSD users to be able to read all the articles in the Knowledge Base.
You'll need a license for every agent that will need to edit the knowledge base, though.
More info: https://confluence.atlassian.com/servicedeskserver031/serving-customers-with-a-knowledge-base-802164866.html
The numbers can differ completely. If, for example, you were to use Confluence for nothing other than a service desk knowledge base, while using Jira for a Service Desk and some process tracking, then you can have:
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