depending on the anime...13 or 26. sometimes an anime has too many meaningless episodes that it would have been better to stop at 13 rather than continue to 26 (peace maker kurogane).
other times i don't have enough time to watch anime and it takes me a while to get through an anime if its longer. other than that...some anime's are nice when they're continued for a long time (unless its like naruto where they have 10+ filler episodes to make naruto look horrible and lame).