How accurate is Ramsay’s “twenty good men” moment in the show? Can a group that small cause so much destruction in a 5,500 men camp?

It’s an exaggeration of how guerrilla warfare can work – what should be happening is Ramsay attacking the baggage train when the army is marching in column, rather than when it’s drawn up in camp, and it should be happening over weeks rather than one night – but yes, guerrilla forces can punch way above their weight.  

On the other hand, this kind of thing can go very badly in a hurry – look up the history of commando forces, and you see just as many unmitigated disasters (the Dieppe Raid, for example) as you do amazing successes.