I have gone through the rules and guidelines for Events on GC.com, and I cannot find anything that stipulates the number of people to qualify for an Event. I would guess that the Event host would be enough to log his/her attendance and that would be fine. The thing is that you do not know who will come/attend an Event that is listed, so you list it and see what will happen. If no one pitches up, then it will be just you that has attended. An Event is just a type of cache, so one visit by a geocacher would seem to be sufficient as far as I see it. So to answer your question - one person attending an Event would be fine to qualify it as an Event!
There isn't a "recognised" or official mini Mega type of Event. So if you have a large Event, it is just seen as a normal Event with a lot of people who have attended. There is a normal Event, a CITO Event and a Mega Event. There are other special types of Events from time to time, like the 10 Year one, but those are quite rare. Only a Mega Event requires a MINIMUM of 500 people to attend and then it is up to Groundspeak as to whether to give it Mega status or not.
I hope this helps?