I have never seen more than 189 passengers in these aircraft. That was dictated by the exit layout. Four full sized doors and four overwing exits. Some versions of the DC-8-50 and DC-8-62 had an additional pair of exits aft of the wing. Called "Jetescape" these were hinged at the bottom with a slide (not inflatable in the DC-8-50 and inflatable in the DC-8-62). The intent of this exit was to allow the overwing exits to be blocked when in a combi configuration.
However, while I have never seen more than 189 in the DC-8-50 even with this exit, I recall low 200s in the slightly longer DC-8-62 but I can not find a reference with regard to maximum with that exit layout.
The cabin volume of the DC-8-10 to -50 would be slightly more than a B737-900 and slightly less than a B757-200.
The exit layout of the DC-8-60/70 varied by original owner. Basically they had a pair of full sized doors at the front, two "Jetescape" exits ahead of the wing roots, four overwing exits, a pair of full sized doors aft of the wings, then another pair of full sized doors in the rear. United had an additional pair of Jetescape doors in the rear cabin which eventually were decommissioned.
As that exit combination would allow more passengers than one could physically fit, the maximum number of passengers would be limited by what the airline "risked" balancing comfort with economies, with lav and galley requirements considered.
The cabin volume of the DC-8-61/63/71/73 was slightly more than a B757-300.
Just because I stopped arguing, doesn't mean I think you are right. It just means I gave up!