To increase the number of servings, keep the ratio of quinoa to water at 1 to 1.25. For example, if you do 2 cups of quinoa, use 2 1/2 cups of water.
If your quinoa is not pre-rinsed, rinse it first! This will help remove the natural coating on it which can have a slightly bitter flavor.
Instead of using water, I like to cook it with chicken broth which adds a little flavor to the quinoa. You can also add some frozen vegetables like spinach, kale, or corn.
A note on the water ratio:
I've cooked quinoa in an instant pot hundreds of times and tried a lot of different ratios of water. I've found anything from 1 to 1.5 cups of water per 1 cup quinoa to be okay, and so 1.25 cups of water to me is the sweet spot. It also depends on what type of texture you like your quinoa. If you like fluffier quinoa, add more water. If you like quinoa that has more of a chew to it, add less water. Also, if you are prerinsing your quinoa, add slightly less water (since there will be some residual water on the quinoa).
The water should also be cold or room temperature. Do not use hot or warm water.
You can just keep the instant pot running (no need to release) if you want to go and do other things. I find it doesn't change the texture of the quinoa too much. However, if you want intend to do a natural release for longer than 20 minutes, I recommend setting the timer for 1 minute. For example, if you just want to set your instant pot, and come back in an hour, then set it for 1 minute!