The Aula at the VU Amsterdam has 900 seats. With this kind of announcement (and showing up at HN) I guess the seats will be filled quickly. Registration is free, and it is polite and appreciated so the organizers know how many people they can expect, but registration is not required. Anyhow, if you want to attend the farewell lecture: show up early :-)

And it would not surprise me if they relocate the lecture to a bigger auditorium nearby, if interest or registration goes way beyond 900.