Hmmm... Arnold DeLeon brings up a good point.
There's a thing called "frequency shift curtailment" whereby the Powerwall 2 or the backup gateway 2 (not sure which, I suspect the gateway) changes the frequency of your system while in off-grid mode in order to control how much power your inverters are making. This may not be supported by all inverters, but it basically means that there isn't a set ratio that you need to observe in order to maintain power in an off-grid scenario.
So if you're configured for frequency shift curtailment, and making more solar power than your battery can use for charging, and your house load doesn't make up the difference, and the grid isn't available to absorb the difference (that's 4 conditions that have to be met), then the system raises the AC frequency progressively above 60 Hz. The inverters respond to the higher frequency by reducing their output, until they eventually shut off. At that point, your house is running on the Powerwalls(s) alone, until they discharge far enough to call for a charge. When that happens, the system reduces the AC frequency again, the inverters come back on line, and provide enough power to both run your house and charge your battery, provided that there's enough solar available to do both.
In theory, if you have enough Powerwall capacity to carry your home through the dark hours, and have enough solar to both run your house and recharge the batteries, you can run more or less indefinitely until the grid comes back, without resorting to a whole house backup generator. In practice, this gets pretty hard in December and January because there just aren't enough sunlight hours in the day during that time of year in North America.
Our system is built to cover all of our typical usage other than charging the car or running the air conditioner (i.e., for baseline needs) 10 months out of the year. We're net positive 8 months of the year, and break even in November and February, based on average daily consumption. Our baseline needs are small enough that 1 Powerwall can carry our home under normal circumstances for 12-16 hours, so we only bought the one.
YMMV, of course.
The devil's in the details as they say... But there are other options aside from those noted above. It's not just about how many Powerwalls you have relative to your peak solar output as I understand matters.