Unlike grace, we do not need faith in quantity. Jesus used quantitative terms when speaking of faith, but notice how He used them: "Immediately Jesus reached out his hand and caught him. 'You of little faith,' he said, 'why did you doubt?'” (Matthew 14:31).
Jesus equates “little faith” to doubt:
"But when he asks, he must believe and not doubt, because he who doubts is like a wave of the sea, blown and tossed by the wind" (James 1:6).
"But the man who has doubts is condemned if he eats, because his eating is not from faith; and everything that does not come from faith is sin" (Romans 14:23).
Jesus also used the word small to describe faith: "Then the disciples came to Jesus in private and asked, 'Why couldn’t we drive it out?' He replied, 'Because you have so little faith. I tell you the truth, if you have faith as small as a mustard seed, you can say to this mountain, "Move from here to there" and it will move. Nothing will be impossible for you'” (Matthew 17:19-20).
With little faith, nothing happens. With small faith, great things happen.
Jesus had another word for the faith of the Canaanite woman whose daughter was demon-possessed: "Then Jesus answered, 'Woman, you have great faith! Your request is granted.' And her daughter was healed from that very hour" (Matthew 15:28).
“Great” was an expression of the reality and strength of her faith, not the quantity of it.
"The apostles said to the Lord, 'Increase our faith!' He replied, 'If you have faith as small as a mustard seed, you can say to this mulberry tree, "Be uprooted and planted in the sea," and it will obey you'” (Luke 17:5-6).
Why should Jesus increase your faith? You do not need much if the faith you have is real. Look at the story Jesus told immediately after this:
"Suppose one of you had a servant plowing or looking after the sheep. Would he say to the servant when he comes in from the field, 'Come along now and sit down to eat'? Would he not rather say, 'Prepare my supper, get yourself ready and wait on me while I eat and drink; after that you may eat and drink'? Would he thank the servant because he did what he was told to do? So you also, when you have done everything you were told to do, should say, 'We are unworthy servants; we have only done our duty” (Luke 17:7-10).
The last sentence is Jesus’ answer to their request for increased faith. The way to increase your faith is to obey God.