Six on a phone screen
A square collage of six even slots gives each picture about 350 by 522 pixels. On the screen it will actually be viewed on, in a feed, that is roughly a thumbnail. Faces survive it; text in a photograph does not, and a wide landscape reduced to a sixth of a square stops being a landscape and becomes a green stripe.
The four uneven layouts all exist to buy one or two of the six some room back. One large with five around it keeps a single picture at two thirds of the width and lets the other five be what they are — supporting detail. Three pairs gives the middle row extra height, which is the least fussy way to put a little weight in the centre of a set.
Order matters more than it does at three
With six pictures the eye reads the collage rather than looking at it, and it reads in the order the slots are laid out: left to right, top to bottom. Photos land in the slots in the order you opened them, so the first thing worth doing after filling one is trading a couple around. Drag one slot onto another and the two pictures swap — you are moving the pictures, not the slots.
A quiet trick that works for occasions: put the widest, calmest picture in the top-left slot and the closest, busiest one bottom-right. The eye enters somewhere restful and finishes somewhere interesting, which is a better order than the one your camera roll happened to be in.
When six is too many
If two of the six are nearly the same picture, drop one and use a five-slot or four-slot layout. A collage is not an album, and the number of slots is a choice about how much attention each photo gets rather than a container to be filled. Switching layouts costs nothing here — your pictures move across in order and keep their framing — so it is worth trying the smaller one before you settle.