A carousel is the only Instagram layout that keeps its shape. Post nine tiles as a grid and your next post breaks it. Post the same picture as a carousel and it stays intact for as long as the post exists.
The thing that goes wrong is cropping. A carousel forces every slide to share one aspect ratio, and it crops mismatched slides to match the first one. Cut a wide photo into three tall slices and you have three slides in a shape Instagram does not want, so it trims them and your composition loses its edges.
Padding solves it before it happens. Each slice is centred on a 1080 × 1350 px canvas with a background you choose: white, black, a colour, or the blurred edge of the slice itself. A 1200 × 900 slice lands on a 1200 × 1500 px canvas with a band above and below it. Every slide is then the same 4:5 shape Instagram is happiest with, and the crop never fires because there is nothing left to crop.
The slide count runs from 2 to 20, which is the carousel maximum. Three to six is where most wide photos land, because past that each slice gets narrow enough that the padding dominates the frame.
Slides are numbered in swipe order. Upload them left to right so the picture reads correctly, and check the first slide on its own, since that is the one that appears in feeds.
The cutting and the padding both happen in your browser.