1. First Blood (1982): John Rambo (Sylvester Stallone) kills approximately 20 law enforcement officers and National Guardsmen during his confrontation with the local police and military.
2. Cobra (1986): Marion Cobretti (Sylvester Stallone) kills numerous criminals and gang members throughout the movie, with an estimated body count of around 30.
3. Rambo: First Blood Part II (1985): John Rambo goes on a mission to rescue American POWs from Vietnam and kills a substantial number of Vietnamese soldiers. The exact count is difficult to determine, but it's estimated to be in the hundreds.
4. The Expendables (2010): Barney Ross (Sylvester Stallone) and his team of mercenaries kill several dozen henchmen and soldiers working for a corrupt dictator in the South American country of Vilena. The estimated body count for this film is around 50.
5. The Expendables 2 (2012): The team of mercenaries faces off against a powerful criminal organization and kills a significant number of their members. The estimated body count for this film is around 100.
6. The Expendables 3 (2014): The team of mercenaries takes on a ruthless arms dealer and his private army. While the exact number of kills is not specified in the film, it's estimated that Stallone's character is responsible for eliminating dozens of enemies.
It's worth noting that these estimates are based on on-screen kills, and they do not include fatalities that may have occurred off-screen or as a result of indirect actions or decisions by the characters. Additionally, some films in Stallone's filmography may have lower or higher body counts depending on the specific scenes and storylines.
Overall, it is difficult to provide a definitive number, but based on these estimates, Sylvester Stallone's on-screen kills across the aforementioned movies could be in the several hundreds or even thousands.