Can anyone tell how SESSION count is calculated in Google Analytics?

How much time a single user stay on your website. If a user land on your website and spend some time on your website and navigate through your website pages, in this condition user count will be one but sessions count will be more than one.

The total number of session is how much time user stay on a single page. User can increase or decrease session time.

Each time a user initiates a session, a session counter increments for that user. For example, if User A comes to your site for the first time in January, that user's session count is 1. If the user returns in February, the session count is 2, and so on for each subsequent return.

Sessions in Google Analytics tell you everything you need to know when a ... performs in the time of a website visit is counted as a single session.

the total amount of time of all sessions during a given period divided by the total number of sessions during that same time