2004 reverse light LED
 

I am replacing OEM reverse light bulbs with LED white light and much brighter. I don't know the max watts I can use. Any help on max watts per side or gauge of wire?

LEDs use less power watts, they are rated at higher LIGHT watts but lower power watts, so 30watt LED will only draw 3 watts of power, just an example you need to check makers specs

Look up factory bulb replacement to see max wattage supported by fuse and wiring