I believe a session starts on the first visit, even if the user is a "guest. logging in changes the session information to reflect that the user is not a guest but an authenticated user.
I think it works out to 1 user = 1 session. Sessions always start on first visit.
I think that's typical of all session packages.
but it looks like your using Symfony\Component\HttpFoundation\Session\Session
for your sessions, so you should look at the documentation on that.