I'm planning to put lots of phone chargers on one circuit, how to calculate this
0.5A in.  30 of those and your general purpose outlet is at capacity
so you'd need nine 15A circuits to run your collection of chargers.

Now, 0.5A at 110V is 55W  which suggests the charger has a power factor of only about 0.3 

If you need 3KW of charging, start with a power-factor corrected 5V supplies. and connect a bunch of USB sockets to that, you then you only need two 15A circuits.