An "admittance" is someone with a H-1B visa appearing a port of entry like an airport to enter the US. If a single H-1B holder goes on (say) 3 international trips in 1 year, that will count as 3 "admittances" in that year.
The reason why the number is so low in 2021 is that the US government had a COVID non-immigrant travel ban. People with H-1B visas couldn't re-enter the US from many countries e.g. most countries in Europe. Many people in H-1B status (like myself) simply didn't take international trips that year.
