I think there were 10 'events' being investigated (maybe it was 10, or 12 - it gets fuzzy at the edges) - so these 20 fines could have been awarded to just 2 people, if they attended pretty much all of them?
https://news.sky.com/story/partygate-fines-officials-begin-to-receive-50-penalties-following-probe-into-downing-street-parties-12579576