Answer
On Apex Legends PC, open Settings, go to Mouse / Keyboard, and find Mouse Sensitivity.
The EA official PC settings article and accessibility features article both treat mouse sensitivity as a PC setting. There is no fixed recommended value here — use it as a starting point and adjust for your DPI and aiming style.
Caveats
The right mouse sensitivity depends on DPI, screen resolution, aiming style, and hand movement. A fixed value is misleading.