How many known planets are in the Milky Way?
About 3,500 exoplanets have been discovered as of April 2017.
As of April 2017, NASA has confirmed about 3,500 exoplanets. There are also over 4,000 candidate planets which hasn't yet been confirmed.
The web site zooniverse.org has started a Exoplanet Explorers project where volunteers look for patterns in light curves which indicate that a planet has transited its star. In a few days about 200 planets were discovered this way.