It depends on the time period. Thousands of years prior to the movies, there were more Sith than Jedi. By the time of the Knights of the Old Republic games, there are more Jedi than Sith. The numbers then about even out until 1000 years prior to the movies, when all of the Sith except for Darth Bane are wiped out; Bane then creates the "Rule of Two" that keeps only two Sith in the galaxy at any given time. When Vader and the Emperor are killed, Luke rebuilds the Jedi Order and the Jedi then significantly outnumber Sith; by Legacy, over a hundred years after the movies, the Sith are back on the rise to probably equal the number of Jedi still alive.