What number is λ?

λ (lambda) isn't a specific number it's a variable that represents different values depending on the wave or system being analyzed. Each wave has its own wavelength value. For instance, green light has λ ≈ 550 nm, while AM radio waves might have λ ≈ 300 m. Lambda is a placeholder symbol whose numerical value must be calculated or measured for each specific case using appropriate formulas like λ = v/f.