...
At the time, my guess was 13. But that's no more or less likely ...
No, I think "13" a highly-likely number, one of the "mystical" numbers, and usually an "unlucky" one in many Western traditions.
OTOH, it's a very _predictable_ number (for the above reasons).
From the Doylist POV, it suffers from being both
too many (that many same-y foes is narratively pointless, messy, and confusing) and
too few (a reasonable number to actually track them all down, but having "another Nemfected" to produce at-need is very useful for an author (q.v. Justine) which isn't possible if you've already ID'ed all 13).
By this analysis, I like 666 simultaneous Nemfections as the limit: same advantages as 13, without the downsides.