Risk of Rain has a selection of two or more maps for every level and never has a level repeat in the same run, unless chosen to go to previous maps once reaching the Divine Teleporter on the fifth level, Temple of the Elders. Alternatively, should you teleport to Boar Beach, upon completion you will be teleported back to either the Sunken Tomb or Ancient Valley maps.
Each map has two to three different layouts except for boar beach and the final level, Risk of Rain (Level), which has only one layout.
(I don't know where to put this) It should be noted that when further along in the game (2+ hour runs), when in the last second of the portal timer a boss spawns and you move to the next map, the other 3 (they always spawn in groups of 4) will spawn in the next map. This happens regardless of whether they should be able to spawn in that map or not.