Error unable to save photo edit

Only add 1–2 stickers at a time → Save → Re-edit → Add more

This always works, even with 10+ stickers:

1. Add 1–2 stickers
2. Tap Done → Save as New Photo or Save
3. Re-enter Edit → Markup
4. Add the next few stickers
5. Save again

Repeating this avoids triggering the render-failure bug.

Interestingly in screenshot edit you can add however many stickers you want to add and save without a problem.

This suggests that the photos you want to add the stickers to are having a much higher resolution than the screenshots and are larger files. Adding a sticker will add an additional level of transparency, and this way the edited image will get larger and larger and need more and more memory. Saving as a new photo will flatten the additional layers and save memory, so you can continue editing.

You should be sure to report your experiences to Apple personnel in addition to here in this user-to-user forum. We users can only supply advice and bypasses like the one azaksalmarzur28 offered above.