Depends. Some animes need more than 12 or 24 eps for their story to bloom fully. Like Monster. Nichijou and stuff doesn't care about story so any amount can be fine. Other animes would just start dragging out the story and developments unnecessarily, get fillers or simply loose in quality cause they want more, more, more eps. Like most endless series. But it also depends on the guys who make the anime.
Sometimes I feel like watching a short series cause I don't have the time/motivation for longer ones, same with manga/manwha. And at other times I just don't care. :p