How can a 11 bit address bus access 1M memory locations?
The datasheet describes how to select bank, row, and column:


  The Bank Active command is used to activate a row in a specified bank of the device. […] The value on the BA selects the bank, and the value on the A0-A10 selects the row.
  […]
  The READ command is used to initiate the burst read of data. […] BA input select the bank, A0-A7 address inputs select the starting column location.


So there are 21+11+8 = 220 possible addresses.