I have people ask me which lenses I use for a particular shot. I don't necessarily have a specific focal length that I always reach to-- really depends on the situation, but most importantly what I like to refer to as the 'working distance' to a client. How comfortable you are as a photographer with people, and knowing how to pose them in a way that appears natural and inspires confidence is really the key element.
For some people that might be a 24-70 zoom lens, an 85mm prime, or a 70-200 telephoto zoom. All of these focal lengths can take amazing photographs if you capture the right moment. Having quality equipment, maintaining it, and knowing how to use it properly to create the aesthetic you are going for is mostly experimentation in the beginning, but once you familiarize yourself with your gear the next step is to create a connection with your subject.
Personally I like to work at a very casual 'talking distance' with my clients. That means I am at a distance where we can have a natural conversation without either of us speaking overly loud, or missing what the other person said. For me this can be in the 35mm to 50mm range.
When I first started out taking portraits I read articles online advising the 85mm and/or the 135mm focal lengths as perfect for portraits. Naturally curious, I tried both lengths and quickly learned that although they can both create amazing images due to compression (Google lens compression if you don't know what that is), I was so far away from my subject it felt very impersonal.
I'm not saying you should never use those lenses-- that would imply removing good tools from your toolbox. What I like to do is start with a focal length that creates a healthy starting 'working distance' with your client. From there learn to intuitively gauge the distances you can both work at comfortably, find the crop distances that your subject likes, and experiment. I might start with a 50mm and find that I could move to a 35mm or backup to a 100mm depending on the crop and how much rapport I have built with my subject.
There's no baseline, right or wrong method here. Everybody is different and how comfortable you make them feel is really going to make the biggest impression on their images.
That being said, if I could only have one lens it would be a 24-70 f2.8 zoom :)