BOOLEAN FUNCTIONS: generate a function $f: \{0, 1\}^n \rightarrow \{0, 1\}$ from a $n \times n$ S-Box
Your n x n S-Box can be seen as a set of $ n $ boolean vectors of size $ n $. Actually you have got $ n $ functions, each one is the truth table of one particular vector (or column). 
In your example there are 2 columns thus 2 functions. For each column the number of 1's is 2.