Well there will be the COVID-19 episode which won't contain an eviction (I assume), and there will probably be some shitty staged series recap episode before the finale like BBUSA does every year. So that's two of the "extra" episodes filled. Plus they could also do an entire episode dedicated to cut/unreleased/online-only footage as well.
But whatever their plans is to fill the episodes are, it seems quite clear that the season will have 24 episodes because the NZ broadcaster has updated their rate card weekly since first releasing the one that included Big Brother in May, and they haven't removed any episodes at all.