One 24" uvb is perfect. Put it on the left or right side. Your choice, but make sure it's paired up on the same side with bulbs that produces heat such as a Halogen or incandescent. They are a team! A double dome fixture with two 75w is a good starting point but if you find yourself dimming it too much because the overall ambient temps are just too high, try 50w instead. These bulb produce helpful infrared A and B this is the part of the sun that is warming.
You are basically trying to replicate a "patch of sun" on his basking area that he can run in and out of as needed. I recommend two basking bulbs because one doesn't fully cover the whole animal and its not as ideal..
Thrive Dual Dome Lamp at PetSmart. Shop all reptile bulbs & lamps online
www.petsmart.com
Also please remember these animals come from the bright dessert. It's minimal in my opinion to have everything mentioned thus far, but it is really quite dim in their eyes... if you can swing it I highly recommend a 36 inch led light bar from Arcadia placed on the basking side as well, overlapping into cool side. If you can swing it they also have a 48 inch one too that will cover the whole top perfectly.
Arcadia Jungle Dawn LED Bar 51 watts (34")
www.lightyourreptiles.com
Summary: lighting is a team effort!
Player 1= Halogen IRA IRB heats animal
Player 2= T5 UVB and some UVA prevents bone disease
Player 3= LED visible light spectrum promote basking and more energy, better mood.
There's actually a Player 4 too. But it's hard to achieve in captivity at this time..with what is available to most in my opinion so I leave that out.
Here's a great video on the subject that has really helped me..