Currently there are 24 satellite operations as part of the GPS satellite constellation, which is orbital the earth with an orbital radius of ~ 16,000 miles (from the center of the earth). The satellite orbit is placed on 6 orbital planes (the orbital plane is the plane surface formed on the path that is monitored by the satellite orbit – it is close to the circular). The six orbital planes are separated by an angle of 60 degrees. Four to six satellites occupy each orbital plane.
In addition to 24 operational satellites, currently there are six orbit satellites maintained as spares, either 24 satellite malfunction operations. Six spare satellites are kept operational for users to receive signals from them just like the 24 satellites. However, the US Government does not guarantee their availability at all times.
Note that GPS satellites have been launched since 1974. Satellites like others have limited lifespan. GPS satellites are designed to provide a lifespan of ~ 8 years. New satellites are launched in space at regular intervals of time to make up for the satellites that have lived their lifespan.
When satellites are launched into the galaxy, they are entered into pre-calculated positions on their orbital plane. Pre-calculations ensure that a GPS receiver located anywhere on the ground and at all times, may receive signals from at least four satellites. This is required because the world blocks GPS signals from its passing. For a GPS receiver to receive a signal from a satellite, the straight line-of-sight line should not have any barriers in between. Since the satellites are flying across the ground, not all satellites can be seen on a GPS receiver located on a ground floor. So, though there are not 30 operands satellites, not all are visible at a single location on the ground, at any time.
For a more extensive GPS knowledge today, refer to websites like http://www.gadgetknowledge.com/gps1.html