nine though people are begin to think that Pluto is a Moon of a planet we've not found yet. Mercury, Venus, Earth, Mars, Saturn, Jupiter,Pluto, Neptune and Uranus. More on the mystery 10th planet. http://www.nasa.gov/vision/universe/solarsystem/newplanet-072905.html
They found it on October 21 2003 Here: http://www.nasa.gov/vision/universe/solarsystem/newplanet-072905.html
Thats a good question No one really knows - Pluto is a bit of an iffy one as it doesnt fit the pattern of the others, but there also could be others futher out, which are impossible to spot from here as they are too far away from the sun to reflect any light back
Dont understand the Erm I didnt say how many there were, and no one knows how many there are out there beyond pluto (which is a debateable one for a planet - though it is officially recognised as one)