The important thing is to stay present moment aware. You can note fast [multiple sensations a moment] or slow [one sensation per moment]. Perhaps start by trying to note one sensation per moment. You can focus on the body, the mind, etc. Then, when your concentration grows, try opening up your focus/awareness to note two simultaneous sensations per moment, then three, etc. Yes, it can be very hard to note more than one sensation per moment AT FIRST. But what you will find as you practice, and your concentration develops, is that it grows easier and more natural.
To give an example of how I typically practice: I start by focusing on the sensation of the breath as they enter and exit the nostrils. After I able to stay focused on these sensations successfully over time, I then switch to slow noting of the sensations on the body. I usually focus on specific parts of the body [for example, the top of my head, my earlobes, my big toe, etc.]. This type of noting is very focused and restricted. Eventually, my focus/concentration grows, and I open up to include more of my sensations. For example, I will try to note BOTH of my earlobes at the same time, then perhaps my entire torso. Eventually, I open up to try to include my entire body at the same time. This can feel like being covered in pops and fizzles and itches and electrical charges and vibrations all roiling over you like a cascade. Then I try to open up to include my entire body AND the environment around me [sounds, smells, etc.]. Then I open up to include my mind, my body, the environment, etc. Eventually, after much practice, you can develop your focus and concentration to the point where you can note LOTS of sensations at the same time. That said, start small -- one sensation per moment. This will be enough to occupy you for some time. Once you feel like you are solidly staying present moment aware, experiment with opening up: the front and the back of your head at the same time, both big toes at the same time, both arms at the same time, etc.
