Why can I schedule only 10 photos in an Instagram carousel when the app allows more?

You're probably hitting the scheduler's limit, not the normal Instagram post limit. Instagram lets you add up to 20 items when you publish directly in the Instagram app, but its publishing API caps third-party schedulers at 10 items. So if you're using Business Suite or a scheduling tool, it can't auto-publish a 20-slide carousel yet. For more than 10, make the post directly in Instagram and use its own scheduling option if it appears for your account; otherwise, split the carousel or set a reminder to post it manually. Switching to a different scheduler usually won't change that API cap.