Im on my 12th month now and I have two sites. I plan on making another one in a different niche in the future, probably a few months, so I anticipate that I will probably have 3 in my first 18 months.
I heard that its easier to make $1000 of 10 websites than it is to make $1000 off one, so I kept that in mind and started to branch out.
Although, I would never make a half-assed site. I like to keep all my sites in check and add content as best as I can, so I think 3 is going to be my maximum for now. I wouldnt want any getting left behind because I have no time for them.