I've seen this line on a website for
Trimpel Oil refinery
By May 1940, reconnaissance Spitfires had begun flying combat missions using the 100 octane fuel. By 31 July 1940, there were 384 Spitfires serving in 19 squadrons using the 100 octane fuel.
There's no reference to where they got the 384 in 19 squadrons from.