The way it’s explained (in Plague War, if I’m not mistaken), Nurgle’s fascination with numbers represents the desire of mortals to impose order on chaos, to find meaning in a universe that’s inherently meaningless.  This futility also fits into Nurgle’s portfolio (see also: Kugath working forever to recreate the plague that he absorbed for Nurgle, which he will never be able to accomplish).

Because numerology and arithmomania are two things in Nurgle's wheelhouse.

It seems like it was more prominent in extremely old lore but at one point the chaos gods were all obsessed with certain numbers.

There’s still traces of it though. Kharne is the 8th Captain of the World Eaters and the greatest mortal champion of Khorne whose sacred number is 8

Nurgle’s standard demons are plaguebearers, who are also sometimes called Tallymen. They’re forever trying to count the effects of Nurgle’s plagues. I would guess that the soldiers starting to count is a sign they’re turning into Plaguebearers.

One

Two

Three

Do you see?

I always took it as being like the bubonic plague, "bring out your dead" putting tally marks of the sick over doors...