How many images are formed if two parallel mirrors are put facing each other?

Very many. The glass of real mirrors is not perfectly clear and the reflective coating of real mirrors is not perfectly reflective so the images dim. But is not hard to get 50 or more spots of light when shining a laser pointer into such a mirror configuration.

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Edit:Actually, when you look at one of these mirrors, the number of images will increase continuously at the velocity "c", so you can accept that its limit is infinite, but in other hand it's finite because the speed of light "c" is finite, it's a physical "optical" paradox.