'legion' means 'many' here. "His list of friends and admirers there is many". This form however is too archaic (out of date) now. You'd usually express it in conversation or writing as: "He has many friends and admirers there. Using 'legion' makes the sentence more formal. You generally would only use this word in more formal types of writing. Only rarely would you find it in conversation - it's too formal and a little archaic.

To the question you asked: "there" in this sentence means "in a certain place/city/country/region" which would have been specified in an earlier sentence. (e.g., "Of course he'll want his tour to stop in Shanghai His list of friends and admirers there is legion.") To the question you didn't ask: "Legion" is what the Romans called a group of 4000-5000 soldiers. The word is used today as a metaphor for a huge number of people.