This LoRA can help generate multiple girls from behind.

It is not necessary to use tags such as "from behind" or "ass", especially if you do not want to focus on the asses too much.

This can be used for groups of 3, 4, 5, 6+, or more, but it may not always be accurate as AI is bad at math.
Generally, the wider the frame, the more people can be included in the image. However, there are still problems with generating images beyond a certain width.

Angles: from behind, from above, from below, full body, upper body, ass focus, from side(only affects pose).

Poses: all fours, lying, on stomach, kneeling, standing, sitting, bent over, leaning forward, from side, etc.

Actions: looking back, ass grab, spread ?????, facing away, etc.

Objects: backboob, thigh gap, back, legs, kneepits, feet, sole, high heels, etc.

Maybe there are other possibilities, I can't test them all.