The congress is a yearly multi-day conference organized at the end of each year by the CCC [0] "Europe's largest association of hackers". Not sure how attendance at bsides style conferences is but I'd figure it's more like Black Hat, somewhere around the 17/18k mark for the last year iirc.
More like DEFCON than Blackhat. Blackhat is a “corporate” conference and expensive to attend, DEFCON and CCC are more community driven and thus affordable.
[0] https://www.ccc.de/en/home
edit: ah yes, DEFCON, thanks. I always get those two mixed up.