How much wine can I bring back to the U.S.?

Not in carry-on.

In your checked luggage.

If you are of age, and California law allows.

Yes you can if you are 21 or older. Assuming your entry is in CA, here's the link https://www.abc.ca.gov/permits/importing.html

Just keep in mind the 50 lb weight limit on the bag, otherwise haul away.

OK good answers, thanks! My entry is in Chicago with a short layover before flying on to San Francisco. Hopefully the California resident rules apply!

It will be Illinois law then, probably Cook county.

Are you asking about customs declaration? One liter per person is allowed into US free, otherwise you are supposed to declare and pay duty on them.

You declare them and have the possibility of having to pay a duty on them. I declare everything that I bring back from France and am often over applicable limits and still have never had to pay. If Illinois law applies to you (which it should not if you are merely in transit), you are allowed to transport wine into Illinois for personal consumption.

Connecting from an international to a domestic flight in Chicago, my experience has been to go through both US immigration and customs procedures in Chicago, the gateway city. I picked up my checked suitcase, went through the Customs inspection and rechecked my bag for its final destination right there. . I don't remember anything to do with individual state laws but I wasn't toting a vat of liquor.

The application of State laws by CBP is one of those hazy gray areas. I have never seen any confirmation that they will enforce anything other than the US Custom regulations. I do believe that if your Port of Entry is your destination, then it is more likely that State officials will get involved and enforce state regulations, but in transit I just find it hard to believe CBP will take the time to review your state laws (meaning they need access to 50+ sets of pages and pages of documentation) to determine if your relatively few bottles of wine comply.

I am in no way implying that you should ignore or try to sidestep your States laws, just giving my observation based on frequent experience and bringing back anywhere from 5 to 50 bottles of wine. Do declare all alcohol and food. It is very rare, almost inconcievable that you will be required to pay any duty, I think the duty amount would need to be in the hundreds of Dollars before they deem it worthwhile. Pack well in your checked bags, watch the overall weight.