24 may sound like a lot but as mentioned above it is only one spare per body for each wedding day.
I have tried a few of the third party options and they have all been pretty crap.
When I shot Nikon I had quite a few third party batteries and they were fine in terms of lasting as long as oem but they they would develop faults eventually but they were so cheap it didn't really matter.
We often have weddings Friday/Saturday and during the busy months we can sometimes have 3/4 weddings a week, we try were possible not to book any more than that as having done weeks before we have shot 6 and 5 it becomes very draining physically. So any time saved can be a huge bonus and I really dislike charging anything overnight so having a lot of batteries really helps. When we get a busy period, my eldest looks after getting everything charged up and ready for us.