Regardless of focal length, portrait perspective is determined only by the distance at which the camera stands, which is important. This could include like not making the nose appear larger than it is. Focal length does of course influence where you need to stand.
It is all opinions, but 6 feet might be considered a minimum. 5 feet can be too close for portraits, and 4 feet absolutely is. Many studios prefer 8 or more feet for formal portraits that they hope to sell. The 105 mm lens was popular for 35 mm film portraits solely because it forced standing back properly. Some preferred a 135 mm lens for same reason (speaking full frame, cropped sensors would use shorter lenses to stand back at same distance).
Focal length and subject distance can vary, depending if it is a head and shoulders shot, or full length or group. But in every case, there is this same minimum distance for proper portrait perspective. Zoom in all you want, but do stand back a bit.
Some users imagine a 50 mm f/1.8 lens is needed for outdoor portraits, thinking to blur the background. The pros would avoid the f/1.8 issues, and would likely use a 200 mm lens to blur the background even better, plus better portrait perspective too.