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The requirements for becoming a horde is that your faction (or the one in question) is capable of it (the Romans, for example, cannot) and that they give up/abandon their last province.
Upon conquering a new province, the faction has the choice of sacking it for money, or settling down, disbanding the horde.
I have never tried to horde again, but the game mechanics seem straightforward- give up that last province to become a horde, settle down to disband it. I would assume from the above that you can horde as often as you want.
Maybe someone who has actually tried hording multiple times can give a better answer.