There are three ways for you to earn more Fills:
1) You receive one free fill once two challenges you took part in end (you will see it in the 'end of challenge' notification).
2) Pressing on the 'Invite Friends' from the drop down menu at the top right corner.
This allows you to invite people to GuruShots and once they join a challenge grants you an additional Fill.
3) Press on the 'Fill' store in 'my challenges' to purchase more.