because a monopoly used to mean that your company had eaten up many smaller companies so now you're one big company. thus, the direct translation of its roots would be, "One from Many"

Mono does mean one, but poly is latin for city it comes from the word polis like indianapolis.

Greek has two letters corresponding to the English O. The word for "many" is "poly" and is spelled with the short O, called omicron. The word "polein" for "to sell" is spelled with the long O called omega. The two words aren't related in Greek. The word "monopoly" means "one seller" and comes from "mono" plus "polein", not "mono" plus "poly".