If you find the main entry point and place a humane trap right up against it, so that they come straight out of their hole and into the trap - baited with Cadbury's Caramel, you may find you catch them all one by one.
Periodically mice reappear and get caught straight away in our home using this method.
We watched the mice for a bit and worked out where they were entering the rooms from under the floor boards - or if you enter a room and see a mouse, watch where it runs to when it sees you and that is probably where it came in. This works a treat for us. They appear to have given up trying to come in now.
I've heard of people sprinkling flour around so that they can see mousey footprints to trace back to the entry point, but we've never had to do this.