According to this source for 20 mm guns:

https://www.destroyers.org/tcs-ships/ordnance-photos-and-articles/20-mm-single-and-twin-mounts/

"Usually, every fifth round in the ammunition belt was a tracer that assisted the operators in spotting their fire."

I could not find anything definitive for 40 mm guns except one source which claimed they COULD be loaded every fourth round with a tracer. Most sources claim every fifth round as a tracer to be a common way to load any automatic gun, but the 40 mm clip was only four rounds, so of they wanted a similar frequency then every fourth round would fit.

I found no mention of tracers in 5"-38 ammo sources, and with proximity fuses, that is not how they were fired in any case. They were not intended to hit directly, but put up a wall of flak, and often at long distances where tracers would not be so useful.

HTH some