There are 120 Dark Souls scattered throughout Liveside and Deadside. Here is the # of Dark Souls required to level up your shadow power.

Level 0 - Your starting level
Level 1 - 1 Dark Soul
Level 2 - 3 Dark Souls
Level 3 - 7 Dark Souls
Level 4 - 15 Dark Souls
Level 5 - 23 Dark Souls
Level 6 - 35 Dark Souls
Level 7 - 51 Dark Souls
Level 8 - 71 Dark Souls
Level 9 - 95 Dark Souls
Level 10 - 120 Dark Souls

The Light Soul is found deep within The Fogometers (or Undercity for old schoolers) after collecting all 666 cadeaux.

Also, most Dark Souls in the Asylum: Engine Block are inside True Forms which can be accessed via the Fives' prism gates, but there are a few that lie exposed within machines.