4 rations are to be received at the end of the day by default. If you decline any of the DAC members requests, it decreases. Or, some characters may ask for a ration to help them out.
Yes, tax does increase ration cost, but there is a limit on how low. However, if you’ve increased the tax previously, and then lowered it again, it will decrease.
Stress will not change the ration amount, only the money you receive. :)