A stonemason cuts and shapes stone for building processes, including building decoration such as columns and pedestals. While a standard mason will work in stone, a stonemason's skill is vastly superior and much rarer.
Stonemasons often teach their children, so the skill is passed down, this inherently keeps numbers low, and only stonemasons that go on to become master masons will teach others.
While players are generally free to select a citizen role as a background, they should check with the Dungeon Master to make sure it fits within the local area, other party members and does not conflict with the intended adventure.
Citizens have proficiencies in weapons and armour relative to their type and region, so it is not included in the above information, and players do not gain any proficiencies from choosing a citizen background.