[dkfellows]

The million was a figure chosen to be eye-catching to funders and to force the initial design team to address scalability from the beginning, according to Steve Furber. So yes, it's a bit arbitrary.

The actual figure is (for technical reasons) a multiple of 2592. Those technical reasons? That's the topological tiling unit used in the overall toroidal mesh (48 chips per board, arranged to tile in groups of three boards, all times 18 which is the number of cores per chip; 1 OS core, 16 application cores, and 1 bonus that is sometimes available and sometimes not, in order to keep overall chip yields sufficiently high).