It is worth reviewing this page https://confluence.atlassian.com/servicedeskcloud/managing-access-to-your-service-desk-732528940.html if you have not already done so as it describes the different ways of managing access.
Essentially a user who only creates tickets in service desk does not get setup as a JIRA user but they will only have access to the service desk view of the ticket. In this way you have unlimited Service Desk users regardless of your JIRA licence.
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