Brian,
Like all the other car show swap meets, the last 5-10 years have seen a downward trend in vendor attendance. You still never know what might show up there in those parts isles. I think the are six or seven rows about 1/8 - 1/4 mile long. They used to have every space filled with vendors. These last few years have seen lots of empty spaces by past comparison. Friday is best for parts hunters to get the best finds, although getting an early Saturday morning start isn't terrible.
Despite the decreasing size of the vendor area, the Carlisle Ford Nationals show, seems to just keep getting bigger every year. This place is a massive event for all things Ford. I don't really know the numbers, but figure at least 3000-3500 cars and trucks, all built by Ford. By comparison, the GM event only a few weeks later isn't even close to half the size in both show cars and vendors. Here's a photo I put together from the north west corner. This doesn't even show the vendor area at the southwest corner - to the south east corner that is behind the grandstands in the distance. It also doesn't show the entire truck and Mercury / Lincoln area to the east and north, up the hill.