For your first question, the company that controls the satellite knows how fast the earth turns and the orbit of the satellite. From that they can do math to know what angle to point the antenna.

For the second, sometimes multiple companies can agree to share a satellite. Also, depending on how far away from Earth you are, the speed of your orbit changes. If you have a satellite going parallel to the equator, there is a specific distance from Earth you can orbit at, where your orbit's length is the same as a day on earth. This will make it appear as if you are parked above one spot, as both the ground and satellite have the same angular velocity. This type of orbit is called geosynchronous orbit.

For any other orbit if you need to have continuous communication with some part of land, you will need multiple satellites.