Might need 1,000-10,000+ gallons of water to extinguish a structure fire (lots of factors in building size, construction etc) 

Then add water needed to protect other structures from catching fire can easily run up to 10,000 gallons per structure with 30-50,000 structures may need upwards of additional 500,000,000 gallons of water. 

Then add water needed for structures that are on fire that's another many millions of gallons.

Then add the water needed to combat the 38,000~ acre Wildland fire which can be between 5,000-25,000 gallons per acre. 

Literally could be upwards of 1 Billion gallons of water needed. 

Palisades, Eaton, Hurst fires literally need the water equal to the entire yearly water dropped by LAFD aerial operations in 2020 and entire yearly water dropped by LAFD helicopter operators in 2021 in a week versus 2 years of water used in aerial firefighting.