Definetly buy another one--else she may feel left out,
i'd say either one female, or one female and a male.
depending on the space in your tank, and if you would like tons of babies!
yes add a few more..females and maybe 1 or 2 males..keep in mind that they eat there own if they have babies..if yoyu plan on breeding start up a fry tank..maybe a 10 gallon or bigger..and make sure its cycled...the babies of platies are very sensitive to evrything...feed them liquid baby or crushed flakes...very finely crushed..do not DO NOT introduce them into the big tank until they are about half an inch to an inch long..as they will be eaten if to small