Press, hold, and drag iMessage apps to reorder them – you can move them from the lower section to the top section to prioritize your most used apps
On an iPhone 14 Pro, after moving apps from the bottom to the top screen I was able to fit 10 apps (11 including the More button)
To add apps, after tapping More, choose Store to add new iMessage apps
At this time, the Store can’t be moved from the bottom to the top section (or at all)
In the fourth beta, there doesn’t appear to be a way to delete apps inside Messages – hopefully, that will come soon. For now, you’ll have to head to Settings > Messages > iMessage Apps