Based on the year of the song...and the last time Chryslers made “whales”...suspect the 1971 to 1973 New Yorker....the seating twenty line is probably hyperbole.
I had a friend with a 1971 Cadillac Eldorado. It wasn’t 20 but we would occasionally squeeze 12 in there. 4 in the front the rest in the back on laps. We were much skinnier 16-18 year olds back then. Fun times.
We had a 73 New Yorker. Didn’t seat 20, but would seat 10 I bet.
I took drivers ed in 1970. I grew up in Flint, MI and we had a drivers ed track sponsored by local Buick dealers. No one liked driving the Le Sabre because it drove like a boat.