is there a function that only returns individual numbers ?

I'm not sure what you mean

f(x)=6   only returns one number  (it is the equation of a horizontal line)

or

f(x)=6     where x=5     is only 1 point.     ie  (5,6)

or

\(f(x)=x  \qquad   where  \qquad x\in Z \)

That will return a set of integer points.

I mean , a function that when you pass any number , it will return individual one - like if x = 1 or 4 or 5 or 12 it always returns 1,3,5,7 .... 

I mean , a function that when you pass any number , it will return individual one - like if x = 1 or 4 or 5 or 12 it always returns 1,3,5,7 ....