There are four Lethal Weapon films in the main franchise.

Films in order

• Lethal Weapon (1987)
• Lethal Weapon 2 (1989)
• Lethal Weapon 3 (1992)
• Lethal Weapon 4 (1998)

If you’re also counting related adaptations, there was a TV series (2016–2019) that rebooted the concept, but the question about films specifically refers to the four theatrical releases.