This chart shows the exports in mmt, the US is the largest exporter of corn followed by Brazil and Argentina.
Ukraine is right up there at the 4th largest exporter in the world and Russia is much smaller, about 1/9 the size of Ukraine
This was the estimate prior to the Russian/Ukraine war and Ukraine was expected to export 33.5mmt. That’s over half the size of the US exports and that’s why this is such an important factor when it comes to the global S&D for corn and therefore US corn prices as well.