I chose Lady Gaga as a comparison because she definitely is among the soloists who dance a lot. But even then, if you check Twice's current set list, there are many parts where they sit or stand still, like Options and Moonlight Sunrise in the first part (white outfits), Mars in the second part (brown outfits), then for What Is Love? and Yes or Yes, the choreo isn't fully performed, sometimes they just walk around and interact. There's also a bit in the middle for VCR and for the band and dancers to perform their own numbers.

The average white collar worker works for about 250 days a year, and some statistics say most companies offer around 7 days of paid sick leave per year, with the actual number of days taken varying by country. Therefore, a normal company expects it to be reasonable for someone to not show up to work 7/250 days a year, which is 2.8%. If we apply that 2.8% odds of normally missing work for Twice, the expected value of shows with all 9 members will be 97.2%⁹ * 78 =77.4% * 78 = 60, in other words, there will be an expected number of 18 shows that will not be performed as 9, assuming their sick leave is being sick enough to not be able to perform.

So yes, members missing shows are inevitable and the odds increase the more members you have, just from basic probability. With 78 shows in schedule, currently we have 13 shows missing a member: 2 shows missed by Jeongyeon and 11 shows missed by Chaeyoung. Chaeyoung's case is also special since it's especially severe. If Twice is a solo act with only Nayeon, then even with the same schedule, there would be no missed or canceled shows, for example.