It's a rough indication if how many sources have that file.

When you request a file, the ultapeers check their info and send 10 or so results back to you, which is why you often see 11 sources in the # column.

Once the file starts downloading, it collects even more sources ("alternate locations") and tries to download from the fastest 6 or 8 that it can.

So, the # column tells you if the file has enough recent sources to make it a good bet that it can be downloaded.