The Greenwich lies on the prime Median and shows the east and west direction and it measures the north and south of the equator and is defined in terms of the parallels to the greater circle as the equator.
There is no set number of "lines" of longitude. You're free to draw as few or as many 'lines' to mark the 360 degrees of longitude on the Earth's surface.
However many lines you decide to draw on your map or globe, they ALL cross the equator.
