When justifying the Red Wedding, Tywin mentioned killing a “dozen” at dinner. How many people were actually killed in the RW? The army outside in the tents were killed, so it has to be a significant amount, no?

Thousands. In addition to the men murdered in the tents at the same time that Robb and Co. were being murdered inside the hall, you also have to include the thousands of men who died at Duskendale and the Ruby Ford due to Roose’s betrayal and Tywin’s connivance, without whose deaths Roose and Walder wouldn’t have had the manpower advantage necessary to get them to sign off on the massacre.