Minecraft wiki entry for structure_set, associated text:  structures: The structures that may be placed.
A structure to be placed.
 structure: The namespaced ID of the configured structure to be placed.
 weight: Determines the chance of it being chosen over others.
 placement: How the structure should be placed.
 type: One of minecraft:concentric_rings or minecraft:random_spread.
If type is minecraft:concentric_rings:
 distance: Controls how far apart the structures are. [needs testing]
 count: How many structures to generate.
 spread: How many structures are in the closest ring to spawn. [needs testing]
If type is minecraft:random_spread:
 spread_type: (optional) One of linear or triangular. Defaults to linear.
 locate_offset: (optional) The chunk coordinate offset given when using /locate.
 spacing: Average distance between two structure placement attempts of this type in chunks[note 1].
 separation: Minimum distance between two structures of this type in chunks. Must be less than  spacing.
 salt: A number that assists in randomization; see salt (cryptography).