RE: that youtube video
I only saw 2, maybe 3 armies... Sweeps and Sharkticons (can't remember if he had multiple Wreck Gars as Junkions). IMO buying multiple identical toys alone isn't army building - the toys need to "represent generic or mass-produced characters...with the intent of (using) them as an imposingly large cohesive unit." (TFwiki) - e.g. building an army of Legions Class Vehicons.
I have a few multiples in my collection, but unless that multiple represents a generic/mass-produced character (either officially or repurposed), then I don't consider it to be army building. For example, I have 2 Leader Class Sentinel Primes. Both are opened, one is displayed in robot mode, the other in vehicle mode. I don't consider that to be army building. The only time I've ever bought a second identical toy to use as a generic grunt was a second Generations Scourge. It was pegwarming for ages, so I got it. But with only two Scourges, I guess this doesn't count as army building since it's less than 3.
What about having lots of the same toy-character through owning multiple non-identical toys (i.e. variants, reissues etc.)? For example I own 9 G1 Insecticons, but NONE of them are identical. I have original G1, TFC reissue and the eHobby clone version of Bombshell, Kickback and Shrapnel. I also have 3 G1 Kups -- 1986 metal wheels/toes, 1986 plastic wheels/toes and TFC reissue Targetmaster. Do they count as armies? I'd personally count the Insecticons as an army, but not the Kups.