Each class will help you feel less stressed and more flexible, both good things. And a once-weekly session may also fuel your fire to do more yoga. The value of taking time for self-care and to listen to your body cannot be overstated, and yoga is a great way to encourage both.
The downside to practising yoga once a week, however, is that it's a bit like starting over each time. You are likely to feel sore after every class, for example. You'll feel the benefits of an individual session, but it's hard to truly build your skills, strength, and flexibility this way.
This is especially true when you are new to yoga and learning the different poses. This might make it more challenging for you to motivate yourself to continue, since you may feel stuck in beginner mode.