Can you see how many people are on the forum?
Maybe. I can see site statistic off the main hamburger menu but I am using Discourse on iOS so not sure if it exactly the same as a web browser.
Has number of users and topics etc with active user in the last 7 days but not current.
4500 users total
Go to the Users section under the three line (next to your Profile pic at the top right).
Then this page will show the numbers in the right: