How many artificial satellites travel around mars?

None, since satellites don't "land", they orbit. There are 6 landers and rovers on the Martian surface, 2 of which were lost at arrival. NASA reports 3 types of Martian Exploration missions: (1) Fly-by missions: Mariner 3, Mariner 4, Mariner 6, Mariner 7 (2) Orbiter missions (the satellites): Mariner 8, Mariner 9, Viking 1, Viking 2, Mars Observer, Mars Global Surveyor, Mars Climate Orbiter, 2001 Mars Odyssey, Mars Express, Mars Reconnaissance Orbiter (3) Lander and Rover missions: Viking 1 Lander, Viking 2 Lander, Pathfinder Rover, Polar Lander (lost), Deep Space 2 (lost), Spirit Rover, Opportunity Rover, Phoenix Mars Science Laboratory Rover is scheduled to launch in 2011.