How many hail Marys a day should I say to keep the Devil away?

Receiving Holy Communion would be far more effective.

Repeating something will not help you. Mary may be the mother of Jesus, but the truth is the the more you pray to God the more the devil is around you. So pray when you need God. Start by saying Dear Jesus and pray for others and people you do not know. God knows what you need. He will provide. Have faith in Jesus and learn his teachings. 

As a mother I can see how many people follow Mary to Jesus because of the story with the water into wine. 

But a mother would never want the attention that is to be her sons. He is the true Son of God. So focus your prayers to him and not to Mary. 

God bless

Acts 2:38 

Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.

IDT it will help                   .

None. Mary can’t hear any prayers and she can’t intercede for us humans.

One per breath should be sufficient.

Poor fools, those who think any amount of saying 'Hail Mary....' will do any good whatsoever. It's a mantra that goes against Jesus' warning not to keep babbling repeatedly as the pagans do. There is not one single verse in the Bible that says Christians need to say any particular set of words to "keep the Devil away." But it DOES show the spiritual armor God provides and tells us to resist the Devil, and he will flee.  Catholics need to read Ephesians 6:11-13 and James 4:7 and and 1 Peter 5:9. Then put them into practice instead of their easy-believism routines.

UP TO YOU AND I AM NOT CATHOLIC.

As many as you think will help.

A man was walking down the street one day, when he saw another guy walking along snapping his fingers. "What the heck are you doing that for?" he asked.

"Keeps the tigers away," was the reply.

"But there are no tigers here," said the man.

"I know. Works great, doesn't it?"

Jake will be around shortly to give you the answer, but I believe it's once a day