In my notes, I have mention of "An initial order was placed for 1000x SSFT kits for the C-47 in May 1944".
Again in my notes, I have mention of "a significant number" of US C-47s having been fitted with SSFT beginning in February 1945, just prior to Operation Varsity. Most of the 437th TCG C-47s assigned to the Operation were fitted with SSFT and had the wings vented. A significant number (but not all) of the 436th TCG had also been modified before the Operation.
I have not been able to find out the total number of airframes fitted with SSFT, either prior to Operation Varsity or afterward.
In "Green Light!: A Troop Carrier Squadron's War From Normandy to the Rhine" by Martin Wolfe, he mentions that his squadron - the 81st TCS (Troop Carrier Squadron) - managed to get the last 12 out of 16 aircraft fitted with SSFT just before Operation Varsity.
The British were also supplied with a number of SSFT for their Dakotas, but I have not been able to find out how many or when.