In a #S_n1# reaction the reaction rate does only depend on the concentration of the substrate. Mening that you can add all the nucleophile you wan't but the reaction wont go any faster.
The rate law for #S_n1# is:
#rate_(Sn1) = k[s u b s t r a t e]#
For a #S_n2# reaction the rate law is descriped as the concentration of the substrate and the nucleophile. This means that you can increese one of them, to get a reaction that goes 2x faster, and you can increese both of them and get a reaction that goes 4x faster. Here the rate law is:
#rate_(Sn2)=k[s u b s t r a t e]*[n u c l e o p h i l e]#