I believe there were only three Alfa Romeos racing at Indy:
1) The Bill White Alfa Romeo 8C-35, serial number purportedly 50012, was apparently an unraced spare car at the 1936 Vanderbilt Cup, subsequently purchased by Bill White and rebodied for Rex Mays to drive in 1937 (ret Indy, 3rd Vanderbilt) and 1938 (ret Indy), then Babe Stapp in 1939 (5th Indy), Harry McQuinn in 1940 (11th Indy) and 1941 (7th Indy, with relief by Kelly Petillo), Duke Nalon in 1941 (ret Milwaukee & Syracuse - yes, dirt tracks!), Jimmie Wilburn in 1946 (ret Indy), Cy Marshall in 1947 (8th Indy) and Art Scovell the same year (10th/ret? Milwaukee, ret Langhorne). This car was modified and rebodied to some extent over the years!
2) The Frank Griswold Alfa Romeo B, serial number purportedly 50002, but possibly 50007, a former Scuderia Ferrari car, driven by Louis Tomei in 1939 (12th Indy, with relief by Mel Hansen) and Al Miller in 1940 (ret Indy), sold to Tommy Lee and driven by Hal Cole in 1946 (ret Indy) and Ken Fowler in 1947 (ret Indy) and 1948 (dns Indy).
3) The Wharton-Dewart Alfa Romeo 308, serial number purportedly 422022, formerly owned and driven by Raymond Sommer, was driven by Chet Miller in 1940 (17th Indy, with relief by Henry Banks), sold to Milt Marion and driven by Durant in 1946 (6th Indy), then Lee Wallard the same year (ret Langhorne) and Walt Brown in 1947 (7th Indy), sold to Johnny Mauro and driven by Mauro himself in 1948 (8th Indy, with relief by Louis Durant, 3rd Pikes Peak), Tony Bettenhausen in 1949 (dns Indy), also Mauro again the same year (dnq Indy, ret Pikes Peak) and in 1950 (dnq Indy, 9th Pikes Peak), then possibly Ray Shadbolt (sp?) in 1951 (dns? Pikes Peak), Charlie Bryant in 1952 (7th Pikes Peak) and 1953 (ret Pikes Peak), and Jimmie Good in 1954 (dns Pikes Peak).
A 1938 Alfa Romeo entry for Tazio Nuvolari never appeared, it was said to be for a Tipo 308 (possibly 422022!). Another 308 (Ecurie Naphtra Course, serial number possibly 422044) was entered for "Raph" in 1946 by Harry Schell, but also dna.