единица, двойка, тройка, четверка, пятерка, шестерка, семерка, восьмерка, девятка - are names of digits (symbols of corresponding digits).

+ десятка, двадцатка, тридцатка, сороковка, сорокопятка, сотня, двухсотка и т.д. also can refer to:
- a corresponding number of objects as a whole (as Aluette said);
- a banknote of corresponding par;
- an object, which have a corresponding number in its definition ("тридцатьчетверка" - tank T-34, "сорокопятка" - a cannon of 45 mm calibre).