When you are new to working with artificial light for portraits, you should generally start with a single source until you are comfortable with all the basic properties and techniques. Once you feel comfortable with those, you can begin to embrace more complex multi-light setups. This excellent video tutorial shows a relatively simple two-light setup that creates an eye-catching spotlight effect.
Coming to you from Pye Jirsa with Adorama TV, this great video tutorial will show you a straightforward and compelling two-light portrait setup. You can absolutely create professional-level photos using just a single light, but the beauty of expanding your repertoire to multiple light is the creativity it allows, as you can now move beyond just using a key light and employ a wide range of different effects, which is where you will really begin to develop your unique style and make yourself more able to readily adapt to the needs of your clients and to create a signature portfolio. Check out the video above for the full rundown from Jirsa.
If you would like to continue learning about how to light a portrait, be sure to check out "Illuminating The Face: Lighting for Headshots and Portraits With Peter Hurley!"