[sarasi]

No, one is definitely not enough, and they are not all alike. I have seen hundreds of temples in Japan and still enjoy visiting different ones. Temples vary hugely- have a look on the internet for photos of Kinkakuji in Kyoto (it's a gold-leaf covered temple in the middle of a pond) compared with Kiyomizudera (a large building on the top of a hill with a pagoda and a balcony that gives a great view over Kyoto)- they don't look alike at all. Then there are Shinto shrines, which are different from temples again, although they can look similar if you don't know the distinctions.

Temples are also often famous for what is in their grounds as well as the temple building itself- the Zen rock garden at Ryoanji, the giant Buddha statue in Kamakura, or the cherry blossoms/autumn leaves that can be seen in the grounds of a lot of places.

While "seen one temple, seen em all" is probably a bit of an exaggeration, for people with less of an interest in old buildings, a day or two seeing temples is probably enough for one trip- it's called getting "templed out". It really depends on the individual. I personally think that deer all look the same, and didn't spend a lot of time in Nara Park (which is where I'm assuming the people you know went). The deer are quite annoying actually- you can't sit quietly and eat anything because they will try to steal it, even going as far as to pull plastic bags away from people in the hope that there is food in them- I almost lost my camera that way!