Jira Software: Restricting project permissions
How can I restrict project permissions in JIRA Software?
Atlassian designed Jira Software with sharing in mind. As in default configuration, all the users can view and share all new projects. By default, Jira Software ships with groups and roles with matching names.
When you create a new project in Jira Software, it will automatically assign these user groups to the matching project roles. This gives these distinct groups of users the most common general default permissions for projects.
Check this article to know more about managing project roles.
You can restrict project permissions to your clients for limited access to the instance and they can only see the projects assigned to them. You'll find a detailed explanation on this Knowledge Base article, or you can refer this video tutorial for a step-by-step walkthrough to restrict a project.