Looks like Instagram is testing the ability to add multiple photos to a single post in the latest Android beta build. This feature will allow you to create an album in your timeline, something that could prove pretty handy if the service is your photo-sharing spot of choice.
You can select up to ten pictures to showcase, apply filters separately to each image, and then share them. In theory, your followers will see an album that they can peruse at will, liking individual photos at their discretion. That sounds great and all, but there's just one problem: this experimental feature is very clearly broken.
Getting to the multi-selection and filter sections is the easy part, but Instagram will crash if you attempt to share the album. In Artem's case, the app immediately force-closes upon startup (clearing the data returns things to normal). So the test is obviously still in its infant stages, which is probably why we saw no mention of it in the latest v10.7.0 beta changelog. As usual, we have the APK over at APKMirror if you're feeling brave.
For now, though, Instagram recommends just sticking with using Layout if you want to have multiple pictures in a single post. You could also take multiple photos and share them via a story, if you feel so inclined.