How does phone charger output current relate to input current?
In a switching converter (which 99.99% of phone chargers are), the power going in and power going out is nearly the same, less a little bit for efficiency. So in a buck-converter with 120Vrms input, and a 5V, 2A output, the output is 10W and the input is ~11W (for a 91% efficient converter, let's just use that as an estimate). Therefore, if the input voltage is 120Vrms, the input current is 11W/120V = ~92mA.

So, yes, 20 adapters can deliver more total current than a single wall outlet, but at a lower voltage, so power is conserved (and physics is happy).

As an aside, I don't know if you plan to plug 240 USB adapter into a single circuit, but I don't recommend doing that. Additionally, 92mA is wildly optimistic. Check your USB adapter for its AC current consumption. I happen to have one in front of me with the following specs: INPUT 100-240V~50/60Hz 0.2A, OUTPUT 5.1VDC 850mA. Which implies that plugging 100 of them into a single breaker is enough to cause trouble, among other things.