arrayofboards.cpp

;

const int NUMBOARDS = 6;

int address[NUMBOARDS] = {0x21, 0z22, 0x23 ....}

PCF857 board[NUMBOARDS] = {
    PCF857(address[0]),
    PCF857(address[1]),
    PCF857(address[2]),
    PCF857(address[3]),
    PCF857(address[4]),
    PCF857(address[5])
};

void setup() {
}

void loop(){
    // addressing each
    for(int i = 0; i< NUMBOARDS; ++i) {
        ... board[i] ... // lets you get each board
        ... address[i] ... // lets you get each address
    }
}