There's really nothing on that card that says what 'X' is? I know nothing about the card, but would be amazed if FFG let that slip through!
The full card text is:
"Forced: At the end of the staging step, reveal the top X cards of the encounter deck, adding all enemies to the staging area. Discard the other revealed cards without resolving them. X is the number of players.
If the players defeat this stage, they win the game."