So, in the rule book under horde tokens it states: when stacked they (horde tokens) can use special powers called horde powers.

Am I correct in understanding this to mean that horde powers, at minimum, require a stack of two.

The context in which this comes up, for me, is the Mutant Protester's horde power:

HORDE: GENE NATION: Friendly characters with the Brotherhood of Mutants or X-Men team ability or keyword can use Mastermind, but only to deal damage to this character.

With this power there is no "M" requirement, nor "S" scaling, but it is still a "horde" power.

You are correct in your assumption.