How far apart should my seats be horizontally for my bench?
In my benches, I usually space my seats 3.5 studs apart. It allows ample room for the character's arms, so they don't intersect with your neighbour's body parts.
This is Scripting helpers, my friend. Not Building Helpers. You should decide for yourself how far apart they should be, but it's not the best place to ask that kind of question in here