You can estimate that by looking at EB2 Pending Inventory data of Jan 2009 - Apr 2009 which is exclusively made of EB3 porters. Then you assume what percentage of overall EB3 they could be given how much time & motivaton they had to port from last time when dates moved in 2012. Reverse calculate the overall numbers. Extrapolate that to overall year taking into account it was a recession year. Then account for demand destruction. Finally cross check your numbers by verifying against very old PIs and their ratios to PERMs. Apply those ratios to 2009 PERM data and use that to cross check.
Of course it would be simpler if they had correct PIs.