What’s the threshold for war crimes?

The crimes have a definition with requisite elements in the rome statue.

While many of them do require a certain gravity, viewing international crimes like a more serious version of a normal crime is probably the wrong way of doing it. Some war crimes do not require anyone to die. In other cases thousands could die and it wouldn't be a war crime or crime against humanity because the elements aren't met.

In particular, starvation doesn't require anyone to have died, and it covers more things than just food. Keep in mind its a relatively new crime in international law, it was only made illegal in 1977 (for example during ww2, the nuremburg trials explicitly ruled that sieges were legal). As far as i know nobody has ever been persecuted for it, so the case law doesn't exist, so its a bit unknown.