Side delt exercises with dumbbells

How many of these variants do you actually need?

Realistically one, maybe a second as a supplement. The list looks long, but the lateral raise standing, seated, one arm, both arms and on a bench is still the same job for the shoulder joint. There is no difference here of the kind that separates a squat from a lunge - it is one movement shown in a lot of different setups.

A second pick makes sense when it differs along an axis, not in a detail. The seated raise differs from the standing one in that the legs have no way to add momentum. The leaning version differs in where the resistance shows up. The single-arm version lets you hold onto something and work the sides separately. Two two-handed variants separated only by whether you sit on the bench or stand next to it add nothing new to a plan.

The upright row stands apart. It is the only entry on this list where you can use a weight that means anything, because a bent elbow shortens the lever. The price is that the movement stops being isolated: the instructions say plainly to look for the work in the shoulders and upper back, and not to lean too hard on the biceps.

What does torso position change?

Standing gives you the most freedom and the most ways to help yourself. A small dip and drive of the knees or a nudge from the hips is enough to send the dumbbells up on their own, and it usually happens without a conscious decision, somewhere in the last reps. Sitting with your feet flat on the floor closes that door, which is why the weight you work with seated is a more honest number than the same weight on a standing raise.

The other two positions go further. Leaning out to the side with one hand on a rack upright moves your arm across the line of your body, so the dumbbell has something to pull against from the very start rather than from halfway up; the cost is that one hand is holding on, so you work one side at a time. Lying chest-down on an incline bench takes away the torso's ability to move at all, and the cues for that one come down to keeping the body supported and letting the arms do the work.

Once you hinge at the hips or lie face down, that same arc lands mostly on the rear delt. Hence the entries here labeled for rear delts: the side head still speaks up in them, which is why they show up on this list, but if the back of the shoulder is what you are after, it has its own list in the atlas and starting there makes more sense.