The next twelve thrown weapons or pieces of ammunition you use to hit with a ranged weapon attack deals an extra 1d6 cold damage to that attack, and the target's speeds are reduced by 10 feet (if they have any) until the end of your next turn. The spell ends when all the attacks are made while this spell is active, hit or miss. 

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the number of pieces of ammunition or thrown weapons you can affect with this spell increases by two for each slot level above 3rd.