For a 12   brick thickiness wall (i.e. 30cm nominal thickness). Therefore nominal volume of wall of 20m length , 1m  height & 0.3m thickness 

= 20 x 5 x 0.3 = 30 cu.m.

Normally mortar joint will be less than 1cm, but assuming mortar joint as   1cm thickness, then actual thickness of wall will be = 29cm. therefore , actual thickness = 20 x 5 x 0.29 = 29 cu.m.

Nominal Size of standard brick =  20cm x 10cm x 10cm.

Number of std bricks = 29/(0.20 x 0.10 x 0.10) =  14500

Then,

Number of bricks per cu.m(nominal) = 14500/30 =483.3333
                                                                                =483 bricks.

Considering 5% breakages, wasteges,etc (assume)

                    So,Number of bricks required for 1 cu.m = 500 bricks

RESULT:


        Therefore, number of bricks required for 1 cu.m = 500 bricks .