There are a 13 skulls available in Halo: CEA available on any difficulty, as follows.
[
0:09] - Skull: Iron - Mission: The Pillar of Autumn
[
0:30] - Skull: Mythic - Mission: Halo
[
1:14] - Skull: Boom - Mission: Halo
[
2:06] - Skull: Foreign - Mission: Truth and Reconciliation
[
2:31] - Skull: Famine - Mission: The Silent Cartographer
[
3:22] - Skull: Bandana - Mission: The Silent Cartographer
[
4:03] - Skull: Fog - Mission: Assault on the Control Room
[
4:27] - Skull: Malfunction - Mission: Assault on the Control Room
[
4:59] - Skull: Recession - Mission: 343 Guilty Spark
[
5:43] - Skull: Black Eye - Mission: Library
[
6:21] - Skull: Eye Patch - Mission: Library
[
7:26] - Skull: Pinata - Mission: Two Betrayals
[
8:31] - Skull: Grunt Birthday Party - Mission: The Maw
Getting all the above skulls will also unlock the following achievements: