It sounds like the images you really, really want are some of the ones that were overwritten by the subsequent images you took using the card. At this point it looks like there isn't much of anything left to try that you haven't already done. They're gone.
Is there a limit to the amount of pictures that the recovery programs will find?
Yes. You'll never recover more data than the total capacity of the card. If, over the course of several formats/erasures, you've written 1100 total images to a card that can only hold 900 images that size, then 200 of the most recent images have been written in the same space that was once used for 200 of the images written before you reformatted the card.
One thing you can do is check the total size of all of the files you recovered from the card including the files that had not yet been erased. It's probably going to be pretty close to the total storage space on the card. If that is the case then it is almost certain the files you can't recover were overwritten with newer files. For a more indepth discussion of how this works please see the end of this answer.
I had a similar experience one time when I forgot to put a few extra memory cards in my pocket before a shoot that required parking a good ways away from what I was shooting. I was going to be in a crowd and so left most of my gear in the trunk of my car. About halfway through the shoot I realized I had not formatted the memory card in my camera to erase all of the images from a previous shoot that were still on the card, even though I had copied the earlier images to my computer, done all of the edits, and backed everything up. Naturally it was one of my smaller backup cards that I had used at the end of the previous shoot after filling the larger primary cards (which were half a mile away in my backpack in the trunk of my car on the other side of an entrance/exit gate).
I protected all of the images I had taken that night, erased the rest of the images, and continued to shoot.
I was taking a lot of images because it was at a carnival at night and even though I had the camera on a tripod there were a lot of non-keepers due to people walking in front of the camera during a long exposure, the flashing lights on rides and other attractions only being lit for a few of the colors or only on parts of the ride during the exposure, as well as bracketing exposure and WB for many of the shots, etc. When I began getting tight on space again I decided to go through my images and unprotect the obvious throwaways and then I erased all but the protected images again.
I then shot a few more images and started reviewing them in reverse sequence. When I got to the first of the shots I taken since the second erase operation the next image back was the last image I had shot before the first erase operation.
It was then that I realized I had forgotten to protect the images I had taken between the first and second times I erased all but the protected images in the card! NOOOOooooo! Some of the best images of the night were in that section. I couldn't go back and retake most of them because the carnival was winding down, most of the people had left, and many of the rides were shutting down as they prepared to close.
I got home and immediately copied the unerased images on the card to my computer. I then began a data recovery operation. Since the CF card in question was a Transcend branded card I started with Transcend's RecoveRx application. It found 509 erased images on the card. Most of those were from the two previous sessions I shot using that card prior to the carnival. It did recover 54 of the 82 raw files I had accidently deleted.
(Some kids from a local high school who recognized me from work I had done at their school and begged me to take their picture. Not one of the better compositions that evening but I would have hated to have lost it and not been able to send it to them.)
I then turned to Piriform's Recova using settings specifically designed to recover image files. It identified all 82 of the accidentally deleted files as TIFF files (It always sees .cr2 files as TIFFs). I copied them to my computer and changed the file extensions to .cr2. However, only the same 54 recovered by RecoveRx could be opened by any of my image applications that can open Canon .cr2 files. I can open the other 28 in notepad and get a huge document of gibberish characters, but I have found no way to open them with any type of image recovery software.
My theory is that they were somehow listed in a file directory that was still readable but that the locations those listings pointed to had been overwritten with the 27 images that I shot after the second erase operation. Because the raw files are all slightly different sizes none of the headers of the new images lined up with where the old directory said each of the headers for the old images were located and that is why I can not open them. Should I find a photo application that could open at least fragments of the files I'm pretty sure I would be looking mostly at information from the 27 images I took last and for which I already have perfectly good copies scrambled in with very small fragments of the images they overwrote.
The way flash memory cards (and USB drives) work is that the memory controller on the card assigns different areas of the card for specific directory locations each time the card is formatted. They do this for what is known as load balancing.
Flash memory has a limited number of write cycles it can tolerate before it fails. The number of write cycles each bit on a flash memory card can handle before it fails is very large but it will eventually wear out. So the controller tries to ensure that each storage location of the entire card is written to roughly the same number of times over the life of the card. What this means is that even after you format the card the controller will continue using parts of the card that have not yet been written to until each storage location on the entire card has been written to before it will go back and begin using the locations that have already been used.
That's good for recovery because it means not much is overwritten until the entire card has been written to once. But then it's going to go back and use the very beginning again if that space is showing as empty. When you delete a file normally the space that file used is marked as empty but the state of each bit in that space is not altered. When you format a card pretty much all of the regular storage space on the card is marked as empty and available for use.
Formatting regularly helps the memory controller on the card do a better job of wear leveling. It also allows for better card performance by increasing the likelihood of sequential write operations. If you leave the same file on half the card and repeatedly write files to the other half, erase them, and rewrite other files then all of the wear is going to go to only half the card and the life expectancy of the card before half of it fails will be shorter!
What this also means is that if you have kept a few of the same images on the card over several erase/rewrite operations that each used an appreciable percentage of the rest of the card then the first space to be used after a format will very likely be the space that contained those images that were not erased and rewritten each time most of the card was reused.
