Info may vary, but my statement was during the beginning or "Happy Times" Germany never averaged more than 30 Uboats at sea and did mass damage.
At any one time during the so-called 'Happy Times' for U-boats, there were only ever a maximum of 30 at sea. For an area the size of the northern Atlantic, this was not many. Despite this, they managed to wreak havoc. Individual U-boat captains like Kretschmer were responsible for the sinking of 200,000
{above doesn't include uboats in the med, indian, etc..}
Complete link
http://www.historylearningsite.co.uk/u-boats.htm
The debate as I see it is a comparison to how resources could be best used. Most know high command still preferred the big guns on BB's over Uboats and mass resources were used and we all know the failure of the few BB's compared to Uboats. Imagine early if they would have allocated all those resources to U-boats. If more Uboats would'vbe been built early on, britian may have been brought to her knees.
Churchill also said it was only Uboat damage Britain would have to contemplate surrendering.
To those that argue resources could have been better used, then where would you have used those resources and got comaparable damage? In fact Uboats got lil resources overall and almost brought Britian to her knees..