A 2 by itself almost always signifies a bass suspension.
So 2 is an abbreviation of 5/2. Notice that there is no “six” above the C#. Remember that any raised bass note is automatically a 6 chord (see page 3).
The other common use of the numeral 2 is in a 6/4/2 chord, which is described in the chapter dealing with the numeral 4.
Very occasionally a composer will use 2 interchangeably with 9.