Luke Chapter 17

Luk 17:5 And the apostles said to the Lord, "Increase our faith."
Luk 17:6 So the Lord said, "If you have faith as a mustard seed, you can say to this mulberry tree, 'Be pulled up by the roots and be planted in the sea,' and it would obey you.

This let me remind another similar circumstance, which also asking the Lord to increase the faith. A father in Mark 9 asked Jesus to heal his son who has a mute spirit.

Mar 9:23 Jesus said to him, "If you can believe, all things are possible to him who believes."
Mar 9:24 Immediately the father of the child cried out and said with tears, "Lord, I believe; help my unbelief!"

Although the father has not sufficient faith, Jesus still casts the mute spirit out of this child in the end. Why? Because of the father believes. So as this the disciples asked Jesus to increase their faith, but Jesus answered them, if they have faith as a mustard seed, they can move the tree or the mountain by words. God said, he gives the right to become children of God, to those who believe in His name (Joh1:12).

Many times when Jesus healed someone, he always said "Your faith has made you well".
The point of faith is not focus on quantity, but on if you have.

Mat 21:22 "And whatever things you ask in prayer, believing, you will receive."