Lots! You need to carry a lot of water with you when cycling in hot desert conditions.
During my recent bike ride across northern turkey I found myself often times carrying as much as 6.5+ liters of water on my bicycle. I had three standard bicycle water bottles attached to the frame of my bike and another three plastic bottles (each filled with 1.5 liters of water) on my bicycle’s rear rack. On some days I found myself going through more than 14 total liters of water in a 24 hour period. And still I could have been drinking more!