My perspective is that there is more people today than in 2020, based on the welcome talk (it was full with people lining the walls) and just walking around. They have also opened up new rooms for this year.
Since there is no registration there isn't any hard numbers to go on.
True, it's very busy.
Usually they make an estimate of the number of visitors based on the number of connections to the fosdem wifi.
The main fosdem wifi is ipv6 only. Amazing how many services don’t work on it - Uber was just failing for example. There’s a dual stack one too.
For me it looks as busy as it ever was, although my last time there was in 2016.
FWIW, last time I was at FOSDEM before today was 10 years ago, and it's as packed as (if not more than) I remember.
In general, my observation is that events are, with some variation, back to close to pre-COVID levels in spite of lingering travel hesitancy and some company travel expense restrictions--the latter of which probably affects FOSDEM less than expensive conferences.