When you notice a mice or mouse nest in the house there are usually between 1 dozen to 2 dozen mice in each nest.
So you could have 12 mice to as much as 24 mice in the house if you notice one mouse or mice nest.
However there could be more mice that you may not notice.
If you have some mice in your house you should put out some mouse poison or mice traps to kill the mice before they get more infested.
If you leave the mouse infestation after seeing one mice then you could have more mice being birthed into the house.
So you should stop the mouse problem as soon as possible.