If you don't need to know individual logins but only how many users have been online in a day, it's even easier. The “lastlogin” attribute on User keeps track of when someone was last logged in, so you could easily do a retrieve where the lastlogindate => the start of the day and do a count on that.
Hi Luca,
One of the ways of achieving this is by using a microflow as a homepage, where you can increment a counter before sending the user to a page.
You could then use for example this widget to display the data.