I count at least six landing failures amongst the vehicles that were supposed to land.
- Mars 2. "Crash landed on surface of Mars".
- Mars 3. "Success (First successful landing)". That mission was a failure. It ceased transmission seconds after landing. The mission did not do any of the planned surface science, including use of its rovers, and only managed to transmit part of one rather useless picture of the Marse surface (low resolution and very low illumination).
- Mars 6. "Partial success". Partial success my ***. The Mars 6 lander mission ended with "direct proximity to the surface" at 61 m/s. Stripping away the bureaucrateze, Mars 6 ended with a crash landing.
- Mars Polar Lander. The most likely cause of the MPL failure was software interprations of vibrations that made the engines cut out 40 meters above the Martian surface.
- Deep Space 2. "The probes reached Mars apparently without incident, but communication was never established after landing."
- Beagle 2. "Lost contact while landing; Assumed to have crash landed".
I count 15 lander missions. Of those,
- Two failed in the vicinity of Earth (Sputnik 24 and Mars 96).
- One failed on route to Mars (Phobos 1).
- One failed at Mars orbit insertion (Mars 7).
- One failed at Mars entry (Phobos 2).
- Six failed shortly before, at, or very shortly after landing (above).
- Four succeeded (Viking 1, Viking 2, Pathfinder, Phoenix).
Landing failures accounts for six (seven if you count Phobos 2) out of the eleven failed lander missions.