The police work with and protect the protesters. They also ensure no crimes are being committed by or against the group. The leaders of the protest tell the police what their route will be, so a large police presence is used to block traffic and streets so the protesters can safely walk their route. In the case of counter protesters showing up to start issues, the police are there to keep the groups separated. In reference to the police outnumbering the protesters, it’s hard to tell how big the protest may become. It’ll start with the main group/organizers, and as they walk through DC it’s not unusual to gather more protesters in a snowball effect.