Question on responsibility to hold collective consultation on mass redundancies

Hi there. I had a quick question about large scale redundancies at a UK employer. I've been trying to understand at what point there is a legal obligation for an employer to hold collective consultations when holding large scale redundancies and I've been seeing two things which could mean different things:

1 - when 20+ posts are going to be made redundant, it counts as mass redundancies

2 - when 20+ posts are at risk of being made redundant, it counts as mass redundancies

I was wondering if anyone could advise of which of the two is true? I am expecting a large number of redundancies at my work place but think that it might be the case that less than 20 people lose their job in total (but that for instance maybe 30 roles will be at risk and some merged / people having to compete for them)

Thanks so much