Neon, periodic table Ne, is a noble gas. It's most common isotope(90%) has 10 neutrons for an average atomic mass of 20. But both Ne21 and Ne22 exist and are stable.
Neon is present in the Martian atmosphere as a trace element. It can be imported from Earth until local needs make it cost effective to extract on Mars. In 2022 the Earth total production was 540 tonnes per year. for a 1 000 000 people settlement the equivalent requirement might be about 4000 times less, or about 140 kg per year. The importation from Earth, at 500$ per kg (reference needed), might represent 70 000$ per year.