Edit: I guess I'm not very familiar with California water management. I guess the reservoirs release water when they need to.
And California does seem to be moving ahead with expanding capacity.
https://www.governing.com/infrastructure/california-governor...
Assuming this is a serious question:
If you have 20 half full reservoirs you have twice as much water as with 10 half full reservoirs.
The underlying problem in California is that not enough rain is falling within the state's watersheds for everything people want to use it for. Adding more capacity won't double the amount of water available.
Adding more storage can help if it means less dumping at peaks or if they capture water in other areas. But reservoirs have been running half full for years before the heavy rains this year and last. The root of the problem is not enough water for too much demand.
From (unreliable) memory I think California captures maybe 10-20%.
A quick googling gave this: https://www.latimes.com/local/lanow/la-me-rainwater-lost-wet...
If they build more reservoirs but the rains don't keep up, a lot of it would still be lost to evaporation.