A lot of it depends on what type of portrait you're shooting, head and shoulders, full length, groups, etc.
Traditionally, the 85mm to 105mm (on full frame 35mm format) is considered the ideal focal length. Legendary lenses, the 90mm 2.0 Sumicron from Leica, the 105mm 2.5 Nikkor from Nikon, the 85mm 1.2L from Canon all made their bones as portrait lenses. The slight telephoto compression was considered more flattering on facial features, particularly the nose. Plus, the relatively fast lenses blurred the background nicely for subject isolation and bokeh. If you use a wider angle lens the nose tends to be exaggerated and nobody wants an exaggerated nose, right?
Also the working distance these lenses allowed made for a more comfortable distance between the photographer and the subject.
That's the history, your particular style and subject matter may require you to move away from these standards.
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