LatLngBounds.Builder builder = new LatLngBounds.Builder();

//mMarkers is a List<Marker> with your markers
for (Marker marker : mMarkers) {
  builder.include(marker.getPosition());
}
LatLngBounds bounds = builder.build();
CameraUpdate cameraUpdate = CameraUpdateFactory.newLatLngBounds(bounds, PADDING); //padding between screen borders and extreme right/left markers