My guess is that it will depend on how you plan to arrange the seating and other furniture or focal areas. It will also depend on the style of fixtures and the maximum wattage the fixtures will allow. I've found that in a lot of fixtures that look like they'd be substantial you can't use bulbs with very high watts so they weren't as good light sources as I'd hoped and I had to keep searching. Maybe you could think some more about those considerations and it will give you a better idea of how many.
I think you are right in not wanting recessed can lighting. I don't like them either, except perhaps in utility areas where you don't care as much about appearances and period styling. It sounds like you'll have a lot of natural day lighting, although you don't say what direction the windows and doors face.

kren, my living room and dining room are both about 15 x 17 and each has original, 3 bulb (60 watt max each), ceiling fixtures. They hang down from the 9 ft. ceiling about 1 foot (on a stem) to a colonial pan and then have about 10 inch chains to each pendant globe. There is abundant light for each room. When we had the fixtures restored and the house re-wired, we put a dimmer on the dining room fixture. Soo nice. I would think for an 18 x 20 room you would not need more than a 4 bulb ceiling fixture. I love Rejuvenation for fixtures. You can look up their site on the web, and it has lots of advice about how to judge the size of fixtures a room might need. Remember too, that old houses tended to be less well lit than modern ones. A lot of times, I leave my ceiling fixtures off and just light lamps for the ambiance. Good luck.