Well I can't answer that question for the whole country, but reviewing data for social housing landlords using Entitledto's bedroom occupancy calculator provides an insight.
In the first three weeks of October 18.7% of users had 1 bedroom under occupied, and a further 6.9% had 2 or more bedrooms under occupied. That's over 25% of households completing the calculator that will lose housing benefit from next year.
Obviously these are average figures and show extremes at either end - but overal...
