How many figs is 'an army' to you?

[Sterling Moose]

Enough figures so that it becomes impossible to determine their number at a quick glance.

So true.

[Christian]

When there are too many to paint (for me anyway!).

[Doomsdave]

For me an "Army"= whatever is painted and looks like it goes together.  My sons have a similar philosophy.  "painted" is also a loose term for them.