Minecraft wiki entry for Spawner, associated text: Dungeons
One in the center of the dungeon spawning one of the following mobs with the indicated frequency: zombie (50%), skeleton (25%), or spider (25%).
Mineshafts
Any number of cave spider spawners, densely surrounded by cobwebs, scattered throughout.
Woodland mansions
Optionally one spider spawner, densely surrounded by cobwebs, in a rarely generated secret room on the 2nd or 3rd floor. It is sometimes visible through the windows from outside.
Strongholds
One silverfish spawner in the end portal room.
Nether fortresses
One or two blaze spawners on fenced platforms with full-block "stairs" leading up to them. Normally two are generated per fortress, but there can be fewer.
Bastion remnants
One magma cube spawner hanging from a chain underneath a bridge in treasure rooms.