Step 1: Parametrize the allowed positions. Every point on the parabola can be written using one parameter, say $x=s$, giving $y=ks^{2}$.
Step 2: Since a single parameter $s$ pins down the location completely, the configuration space is one-dimensional.
Step 3: The number of degrees of freedom equals the dimension of the configuration space, which is $1$.
Step 4: Equivalently, starting from a planar particle (2 coordinates) and removing one holonomic constraint leaves $2-1=1$.
Step 5: Hence the motion has
\[\boxed{f = 1}\]
degree of freedom.