Is it better to workout a muscle group ones or twice a week?

Once a week is good enough. Twice a week training can result in overtrained muscles. Overtraining is not good because your muscles won't recover and grow

To some extent, yes, once a week is a good balance between frequency and muscle recovery time. But here's the problem - to do that, you need to hit the gym 5 or 6 days a week so that you get every muscle. And it almost requires that you do isolation movements - weight training exercises that only work one muscle at a time. That's unnatural and time consuming.

The best workouts involve compound movements - like squats, deadlifts, pull-ups, etc. These hit multiple muscles at a time, are safer than machines, and more closely mimic natural movements. But then you have to worry about recovery time...

There are too many factors about muscle recovery time to go into here but they include your age, your fitness level, how intense your training sessions are, whether you are doing body-part splits or full-body workouts, etc.

I've found that for most people, twice a week per muscle is great. If you've been lifting for a really long time (several years) you might find that doing super-intense workouts hitting each muscle once a week is best, but be ready for a lot of time in the gym.