Good points from the other posters. You don't have to have multiple SSO servers for all the sites, but if you are going to, then you would need multisite mode. I should have stated that a little clearer.
As far as should you separate SSO, it is really up to you. If you have really large sites it might be beneficial for growth, but if the sites aren't that large it is probably easiest to keep them on same server and reduce the footprint.