"One likes glasses."
Both "Einer mag Schokolade" or "Man mag Schokolade" would be gramatically correct, but I think their meanings are slightly different. I believe "einer" implies a more precise "one" than "man". You would use "einer" in the sentence: "Ich habe zwei Söhne. Einer mag Schokolade, der andere mag Speck." (I have two sons. One likes chocolate, the other likes bacon.) The meaning of "man" is, imo, more general. For example, "Man mag Schokolade" means that one [generally] likes chocolate.
It means one as in the context of this sentence "One can not go about this quest alone". The word "one" there is in reference to a single, unspecific person, which is not the same a the numerical one. The germans just (understandably so) use to different words for it.
Don't think of words as being a direct translation of other words. Think of what the word defines.