I have a Google analytics account that contains 20 properties with only one login to all those properties.
I'm trying to give permissions to a client for only one of those properties. How do I give them permission for a single property?
You should be able to give user permissions strictly to that single property - https://support.google.com/analytics/answer/2884495?hl=en
You can do this by going to Admin > selecting the property you'd like to share, and then clicking User Management.