Most pleco's are shy, or nocturnal. You can have too many--keep in mind that most will want a cave or niche they can tuck into fully, and they can fight for territory. You also have to take into account that those that are primarily herbivores will produce massive amounts of waste, meaning good filtration and frequent water changes are a must. Mixing plecos with different food needs will give you a few more options, but not many. Currently, my coolwater 55 has too many bottom dwellers--3 whiptails, 3 cories, a weather loach, a miscellaneous ancistrus, a large chocolate, and 5 or 6 otos. The oto's don't really spend much time on the bottom, but every one else does. So, I have lots of caves for them to hide in. The whiptails don't hide, and are probably the most socialable--they are always out, and are very tolerant of me--I can pick them up and move the around. They aren't very active though, spending most of their time hanging out in one spot.
I also have a butterfly pulcher in another tank (not sure if this is the same fish--common names tend to vary widely). He is by far one of the most shy--comes out for feeding, and then darts back into his cave. Gorgeous fish--great colors, cool shape, but seldom seen.
In terms of comfort--I think having lots of hiding places and less competition helps make them more comfortable. All my tanks are planted, with medium to high light, so I try to provide them with shaded areas, as well. This has increased the time my ancistrus spend out--but they tend to be more active fish anyway. Wood in important for many species as well-I have piles in all my tanks with loricaids.