In the past few weeks, I've noticed that virtually all the spam I'm receiving has a number at the beginning of the subject line.

I'm just curious whether this is something Spamcop is adding or something the spammers are doing for some obscure reason...

There are many reasons for this. Spammers add these to the subject line to avoid filters as this makes every subject different by adding something randomly generated to every subject line. Some say this is for tracking purposes. In any event it is just garbage and Spamcop is not putting it there.