zero

According to Russell and Whitehead, zero is the class of all null classes. There's only one null class, so that means that there's exactly one member of the zero meta-class, which makes zero rather unique. One is the class of all classes with one member, of which there are infinite examples. Two is the class of all classes with two members, and again there are an infinite number of these classes, and so on for all natural numbers. All numbers but zero are infinite classes.