Depending on your goals, starting point and physical ability, one personal training session may be sufficient, although 1 to 3 personal training sessions per week are recommended. For those just starting to exercise, 2-3 personal training sessions per week are recommended to ensure they develop a proper shape and a sustainable routine. When you're new to training with a personal trainer, setting a schedule for how often you train can be a challenge. How often you train with your PT depends on what your overall fitness goals are and how quickly you want to achieve them.
For most people, exercising with a personal trainer twice a week is enough to achieve their fitness goals. I have always had a love-hate relationship with exercise. When I like it a lot, in a good place and I feel fit, I love it, but once I fall off the train for a while you're more likely to find me stroking spiders than in the gym. It also depends on what you do outside of your normal PT, E, G sessions that you play recreational sports.
If your goal is to lose fat, bear minimum, you should train at least twice a week with a coach. Within those 2 sessions, the focus should be on strength training to build lean muscle. On other days, incorporating some form of cardio every day will go a long way toward achieving this goal. I have a lot of experience training clients with a wide variety of fitness goals and love diving into the deep end of research on fitness topics.