Ant-Man will have more than one villain, though we don't yet know who they are. We know who will be playing one of the bad guys though, as Avatar and Escape Plan actor Matt Gerald has joined the cast.
This is an interesting move, since Matt Gerald is no stranger to the Marvel universe. He recently appeared as White Power Dave in Marvel One-Shot: All Hail the King, which was a spin-off of Iron Man 3 and featured that film's villain, Mandarin (really an actor named Trevor Slattery) giving an interview from prison. Is Matt Gerald reprising his role as White Power Dave? That is entirely possible. His character is still alive and well, residing as an inmate at Seagate Prison. The same prison that houses Luke Cage, a character rumored to have a cameo in Ant-Man, which will set up his solo Netflix series in 2015. Here is a clip that features White Power Dave confronting Slattery in prison...
Matt Gerald will be joining Michael Pena, rumored to be playing the villain Castillo, a character that is not part of the comic book mythology. No other details about Matt Gerald's character have been revealed, though it must be noted that White Power Dave was created for Marvel One-Shot: All Hail the King and the Marvel Cinematic Universe. He, like Castillo, is not part of the Marvel comic book universe, as it seems Edgar Wright may be setting up an entirely new world for this particular franchise.
Michael Douglas and Paul Rudd are starring as Hank Pym and Scott Lang, with part of the story taking place in the 60s before it transitions to modern day times. Patrick Wilson has joined the superhero adventure in an undisclosed role, as has Corey Stoll. Evangeline Lilly was cast in the female lead, though her character is not believed to be a part of the comic books either. She is rumored to be playing Hank Pym's daughter and Scott Lang's love interest.