Depending on what part of the game's release cycle you are in, the pack distribution for a Sealed or Draft event changes. I'm going to use the most recent block - Scourgewar Block - at time of writing for reference.
Scourgewar, the first set of the Scourgewar block
Wrathgate, the second set of the Scourgewar block
Icecrown, the third and final set of the Scourgewar block
When the first set of a block has been released
Tournament Sealed: 6x Scourgewar
Release Celebration Sealed: 6x Scourgewar
Draft: 3x Scourgewar
When the first and second sets of a block have been released
Tournament Sealed: 4x Scourgewar, 2x Wrathgate
Release Celebration Sealed: 2x Scourgewar, 4x Wrathgate
Draft: 2x Scourgewar, 1x Wrathgate
When all three sets of a block have been released
Tournament Sealed: 2x Scourgewar, 2x Wrathgate, 2x Icecrown
Release Celebration Sealed: 2x Scourgewar, 4x Icecrown
Draft: 1x Scourgewar, 1x Wrathgate, 1x Icecrown
Once Worldbreaker is released, all limited events should follow the 'first set' pack distribution above.
