How many times a day depends on your intensity.
To keep fit scientifically, we must first know our body and what physical qualities we need to strengthen.
For example, the function of cardiopulmonary breathing, scientific data show that it includes (vital capacity, blood pressure, heart rate, frequency). If the data is not high, it means that your function is weak, and what needs to be strengthened is high-intensity aerobic exercise. ~ long-distance running
The speed of information transmission between the brain and the body nerves: if the explosive force needs to be strengthened, it is instantaneous weight-bearing training, such as pull-ups: the explosive force of arm strength, leapfrog and calf.
Oxygen supply capacity of blood: anaerobic exercise, sprint and sprint stop are needed. etc
If you want to gain muscle, that's bodybuilding, not fitness. Muscle growth will increase strength and endurance, but the above items may not necessarily increase.
If it's fitness training, once a day is enough, unless you want to enhance a lot of physical functions ~ ~ ~ If it's muscle training, you need to arrange it reasonably, and you can't be too tired, otherwise it will damage muscle tissue. Generally, if the intensity of muscle training is high, it will be once every three days, and if the intensity is low, it will be once every other day.
It is ok for normal people to go twice a day. In the morning, the cardiopulmonary breathing ability and blood oxygen supply ability, that is, the last 200 meters sprint of 2000 meters.
Do muscle training at night, but the intensity should be appropriate, otherwise it will affect your physical recovery and work the next day.