This all depends on your circumstances. Someone with a short stretch of waterway at the end of their garden will need no more than one smart raft. But a fishing club or a nature reserve would most likely need 2-6, and most Internal Drainage Boards would need 10-30 units in order to clear their waterways of mink. Across East Anglia, an average density of one permanently-active trap per 18 sq km of land has been enough to remove mink, but areas with fewer waterways might need fewer traps. If traps are not permanently active, a much higher density than this would be needed.