I think it depends on the person and their needs and their goals.
What’s their physical state?
What’s their state of mind?
What’s their schedule like?
How much time do they have to invest?
Minimum of twice a week is what I usually suggest, and then add more if you feel able to.
Overall, I’d say between 2–4 a week is ideal.
Every class is different. Some classes are more technical and you can do many classes in a week. Some classes require more cardio and you may need to build up to doing more of those.
Talk to your teacher about your specific needs, and striking a balance between training enough and overtraining or under training.
Starting out, I would say minimum two sessions a week and go up to about four sessions a week. This depends on what the classes are. Some classes are more technical and you can do multiple sessions, while others are more cardio-heavy.
As you progress, try to take as many classes as you can, while still remembering it’s better to do one class fully focused and committed than two classes with less focus and work in each one.
Try to take a variety of classes so you can become a well rounded fighter. You should have familiarity with groundwork, movement, strength, cardio, and weapons.
Even if you can’t train because of personal reasons, you can still watch and observe and take notes.