I assume you've already tried creating small or medium worlds in the regular world generator? I usually play with such worlds for adventure mode, and there are quite a lot of sites. I can easily travel around without companions, as I never have to travel more than half a day to the next site, I so can avoid bogeymen by just sleeping inside.
If you want to generate a really dense population, the perfect world utility mentioned above should be helpful, but I don't know how dense you can actually make it. Due to wars etc the extra sites may just end up killing eachother off. If you figure out how to do it I'm sure others would be interested (myself included).
Edit - I had a go at this, and it seems to be pretty easy. I copied the "small region" preset and set the following params (all others default):
seed - dense
Mineral Scarcity - 1000
Site cap after civ creation - 2000
Max Megabeast Caves - 10
Max Semimegabeast Caves - 20
(Could have also increased Titan Number, but I forgot)
Number of Mountain Caves - 20
Number of Non-Mountain Caves - 30
Make Caves Visible - Yes
Number of Civs - 25
I then ran worldgen for 152 years (I meant to stop at 150, but there was some lag after I pressed ESC). I also ran one with same parameters and seed "DENSE" (all caps) and got more goblins, but only 1 tiny dwarven civ. Here is the result:
This world is jam-packed with sites. I don't know if you have to walk more than 3 squares between any adjacent sites. I'm not entirely sure about the biomes, but there definitely appears to be some variety, and the world is small so it shouldn't be hard to visit everything. Definitely towers, goblins, etc. It's possible that by running longer you can get even more sites, although at some point many will start to be abandoned. A quick bit of adventuring already shows this is a problem in my world, as many of the hamlets I visited were abandoned. Not sure if there is a way around this.
Also, to answer your original question, if you select "Design new world with advanced parameters" from the main menu, then select the size you want and press "e" to Enter advanced parameters, then you can set "End Year" (the default is 1050, so I don't think you had done advanced world gen before).
If you want to do regular worldgen just set the history to change the length of world gen. I think medium is 250 years, and not sure about the others. You can always set "Very Long" and just press ESC to stop whenever you want. Maybe you chose "Very Short" before? Running on a small world with medium history, number of civs, sites and beasts set to "Very High", savagery "high" and mineral occurance "frequent" results in a world even more jam-packed with sites than the one above, although I suspect even more of them are abandoned (didn't check).
It is possible that by messing around with some of the other advanced parameters you can get less abandoned sites (maybe by adjusting population caps?) but I have no idea. Asking in the worldgen cookbook thread is a good idea.