Twenty-one different chemical elements are mentioned in Project Hail Mary.

By frequency they are:

nitrogen 109 times
carbon 57 times
oxygen 42 times
aluminum 22 times
hydrogen 18 times
iron 15 times
argon 10 times
xenon 8 times
sulfur 6 times
helium 3 times
lead 3 times (also 9 other times in other senses)
mercury 3 times (also 6 other times for the planet)
silicon 3 times
gold 2 times
nickel 2 times (also 2 other times for the coin)
thallium 2 times
uranium 2 times
cesium 1 time
chromium 1 time
copper 1 time
iridium 1 times

“silver” is mentioned three times but only ever as the colour.