How to give a client acces to Jira, to only view the Project assigned to this client?
Change the permission schemes so that your Jira does not show anything by default (i.e. restrict who can "browse" the projects), then add your client to a role or group that does allow it for that project.
Have a read through https://confluence.atlassian.com/display/JIRA/Managing+Project+Permissions
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