The naming scheme of Cray X-MP

[Sushil Bhattacharjee]

Hi,
This may be the wrong place to ask this question, but I could not find
any other appropriate newsgroup. 
My question is - What do the numbers at the end of 'Cray X-MP' mean in
the name of the machine? 
e.g.  What does the 12 mean in 'Cray X-MP/12' or the 48 in 'Cray X-MP/48' ?

I do not usually read this newsgroup, so I will appriciate it if you
send e-mail to sushil at pixel.cps.msu.edu, in case you know the answer.
Thanks,
Sushil