Plurals don't work that way in Japanese. While what nhk9 said is right, it can be misleading if "you don't speak a word of Japanese". You can add "tachi" to a group of people, or even a person's name (which denotes they're accompanied by more people). Then again, you can simply say "ronin" and mean more than one.
"mukashi no ronin" could very well mean "The ronin (plural) of long ago". Context can be a ***** in Japanese.