I think this is different for everyone, but for us my 'sleep well at night' cash reserves is $12,000. We arrived at this amount originally because it was the difference between our lowest income earner's annual income and our annual expenses. This means that if the higher earner lost their job, we would have a year in cash reserves. If emergencies came up (our water heater went bust, our vehicle needed an unforseen major repair), we 'borrowed' from our own cash reserves, and then built it up again over time back to the $12,000.
Our reasoning was that it is statistically very unlikely for both of us to become unemployed (very different jobs in different industries), and if that were to happen we would need more major actions than our cash reserves anyways, so we were going with the more possible scenario (even if still somewhat unlikely)
I admit, that number isn't accurate anymore; our incomes have gone up and our expenses not so much, so could be a much lower number now, but I feel very comfortable at the $12,000 so that's where I stay.