Why the three-by-three keeps winning
Nine in a square is the most recognisable arrangement in phone photography, and it works for a reason that has nothing to do with fashion: three rows of three is the largest grid where every cell is still a third of the frame wide, and it is also the largest layout this catalogue draws by hand. At ten or twelve the cells drop below a quarter and the collage starts to turn into a pattern — pleasant, but harder to tell what is in it, which is why no such layout was drawn by hand.
Set the gap to zero and the outer edge to zero and you get the seamless version, which is what people usually want when they say photo grid. Set the gap to twenty and a white ground and you get the framed version, where each picture is its own object. Those two looks come from the same layout and one slider.
The two ways out
A banner over eight puts one full-width picture across the top and eight small ones beneath it — the arrangement to reach for when one photo is the occasion and the other eight are the evening. In a story shape the eight become four rows of two instead of two rows of four, because 270-pixel cells down a tall frame are not worth having.
Nine, unevenly mixes cell sizes on purpose so the collage reads as a page rather than a table. It is the one to use when your nine pictures are a mixture of portrait and landscape, because the varied cells give the wide ones somewhere wide to go.
Nine photos, and why there is no tenth slot
Filling nine slots by tapping each one is slow. Drop all nine onto the canvas at once — from the file browser, by dragging a selection in, or by pasting — and they land in reading order, after which trading two around is a drag from one slot to another. That is usually three or four drags rather than nine separate choices.
Nine is also where the hand-drawn catalogue stops. Past it, the Layout step generates a grid, a mosaic, brick rows or one large with the rest around it for exactly as many photos as you bring, up to thirty. On this page, though, the layout stays at nine: open more and the spare ones wait in the tray under the collage, and tapping one puts it into the next free slot.