I don't see any benefit to creating additional sites since they would all still be running from one installation of mojoPortal there will be no performance difference. The installation would be handling the traffic for all sites so it doesn't matter if you have one site with lots of traffic or multiple sites with less traffic, the total amount of traffic is all that matters.

If you have other reasons besides performance that make you want to use multiple sites, that is up to you, I have no opinion on it. If the content in all the sites is really related and could be in one site then that is what I would do, but if as you say it is about many unrelated subjects, then it seems one site per main subject is the way to go.

Hope it helps,

Joe