The only place that 33 is a "full field" is at Indy. Just for grins I went out to champcarstats.com a picked a year at random out of the so-called "glory days" of CART, 1988, and the average car count was around 24-26. Even the other two 500-mile races (at MIS and Pocono) only had 28 and 26 cars respectively. Nazareth only drew 20.
Nobody in their right mind would put 33 Indy Cars on the track at Milwaukee (there were 22 starters in '88, by the way; Roberto Guerrero was entered but crashed in practice).