10 skulls are needed to join one match of each.
Skulls are gained by:
2) Redeeming a TORCH Code
4) Redeeming Achievements from Events
5) Claiming prize from weekly Guild Warfare
Skull capacity is capped at 200 in the beginning but can be increased through advancement of the VIP system. Note you can also pass the cap with Points 2-5 but not able to gain from fights.