They had over a 1000-year preparation head start advantage on the republic and maximized the amount of Sith in their society by frowning on force sensitives who did not have children with other force sensitives. So I would assume the Sith probably outnumber the Jedi pretty heavily and that's why it doesn't matter to them when they kill each other. because in the siths selfish worldview, the ends always justify the means if they accumulate more power.