You only need 1 kite and 1 board, but that will restrict the amount of days you can get out. If the wind goes up or down a little, you'll be sitting on the beach for a while. Typically most people have 2 or 3 kites and some people will even have a 2nd larger kiteboard for light wind days.
This is the average amount of days you can get out with specified # of kites:
1 Kite 60% of kiteable days
2 Kites 80% of kiteable days
3 Kites 90% of kiteable days