IIRC this is the total number of deaths in the construction business in Qatar over a period of ten years across the important immigrant groups. It doesn't include all workers that died, but it also includes things that have nothing to do with the world cup. The number is not directly connected to the world cup, though it does say something about the conditions your average migrant worker in Qatar works in.

Several organisations report different numbers. The Qatar government reports up to 40 deaths, FIFA claims three deaths (an earlier number from the Qatar government), the International Labour Organisation claims a number around 50 per year but doesn't count COVID and heat related deaths (and working in the desert makes the latter cause of death quite relevant), so there's no real way of knowing how big the problem really is. People who speak out will at least lose their job, though their fate is probably worse in many cases, so there's no reason for any of them to keep track of any real numbers.

The slavery and abuse situation is probably much worse than whatever the actual amount of deaths for the world cup adds up to, especially compared to the death toll in the construction industry for the third world countries these workers come from. People's passwords get taken and only given back after they've done whatever bullshit "contact" they signed, or sometimes there's not even a contract at all. It's not just construction, though; countries in that region of the world are known to employ all kinds of slavery when it comes to immigrants, ranging from maids to store employees to hard labour.

The mainstream media over here has been reporting about this for months and they keep repeating that 6500 number even though there's no good source to back it up. The original source is more nuanced than the media that copied the number make it seem: it's merely the amount of deaths since Qatar got picked as the host, based on data from the countries these workers came from.