One picture with six photos in it travels better than six separate images, whether it is going into a message, a listing or a report.
Photos jammed edge to edge read as a mistake. A consistent gap, even a small one, makes the same set look arranged.
A white background suits almost everything and prints cleanly. A dark background makes bright photos pop but shows any dust or edge artefacts in them.
Each photo is cropped to a square to fill its cell, so nothing is stretched. Crop the important part into the middle beforehand if a particular photo matters.
As many as you like. Choose how many go across each row.
Yes. Each is cropped square to fit its cell, never stretched.
No.