MB4 234a: What is the Shiur of Food for an Eruv Chatzeros? (368:1-3)

How to "correct" an eruv when the food went lower than the required minimum amount. When adding food to the eruv do you have to notify the residents even if it's the same food?

What if the food went below the minimum shiur before the first Shabbos (i.e. the eruv was never used)?

Can one make an eruv on behalf of someone whose house opens to more than one chatzer?

How much food is required when creating an eruv chatzerot?