If you login both with IE and Firefox you get two sessions. If you then login also from another place, you'll get another one and have 3 altogether.

Any user who is not logged in is "anonymous", so the number represents all created session cookies for users who have not logged in.

Is is per day? It seems like they could all be per day except the anonymous number would be huge then.

No, it's all open sessions - and the session timeout is really long (about 10 years).

Blueyed:

Does it matter if the number of anonymous sessions is more than 200.000?
Does this use server's additional resources?

sure, because it's a mysql table and the more entries it has the more space it needs, and the slower it (may) get to insert new ones.
There's a "prune old sessions" setting somewhere, which you should use - set it up as a cron job, so it does not get run on every page hit. (there should be a link to the manual from the setting in the backoffice).
Of course, with a busy site, you may get those 200.000 hits in a few days already.
It all depends.

Well it is a very busy site. I'll prune it anyway.

Thanks for your help!