YesToTheTruth: How many sins or what sin do you have to commit to lose your salvation?

A person's life is in God's hands. Because Jesus loves those who are his, He is merciful with them. He will give correction as needed.

Revelation 3:19
As many as I love, I rebuke and chasten. Therefore be zealous and repent.

Only Jesus knows what time frame He will allow from the moment a person commits sin and that person repenting to get right with Him again (if possible - it may not be.) "Be zealous," to me, means "Get on this right away. Clean up your act fast and don't go back to that way of sin that leads to death."

One deliberate sin is enough to tell God our heart is not 100% with Him. Remember the greatest commandment: "And you shall love the Lord your God with all your heart, with all your soul, with all your mind, and with all your strength" (Mark 12:30). For instance, how many times a married person can be unfaithful until his/her spouse sees a problem in that? I think just once and that one time can be devastatingly serious, especially if for any reason there is no window open to forgiveness. It means "the end." Again, there may not be forgiveness available. It's not for a man to provide comfort to anyone who has committed or may be contemplating to commit any sin presumptuously. God is merciful but He can't be mocked. I don't think someone who has been saved and becomes a serial sinner stands a chance, but I speak as a man. Depending on God's disposition toward what He sees in someone's heart, mercy may not be granted. It's foolish to play with a point of no return, however close or far it may appear to someone's mind.

Hebrews 6:4-6
4 For it is impossible for those who were once enlightened, and have tasted the heavenly gift, and have become partakers of the Holy Spirit, 5 and have tasted the good word of God and the powers of the age to come, 6 if they fall away, to renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame.

So, I would fear for anyone who got saved and then would fail to be sober and vigilant, get deceived by the devil and get caught in his trap. Hence the Bible is full of warnings to believers:

1 Peter 5:8
Be sober, be vigilant; because your adversary the devil walks about like a roaring lion, seeking whom he may devour.

We are indeed talking about willful sins that lead to death (1 John 5:16).

Hebrews 10:26-27
26 For if we sin willfully after we have received the knowledge of the truth, there no longer remains a sacrifice for sins, 27 but a certain fearful expectation of judgment, and fiery indignation which will devour the adversaries.

God will look at someone as an adversary when it is a case of willful sin. Because the Gospel is a path given to sinners who are enemies of God (Romans 5:6-11) to be reconciled to Him and be his friends who do his will on earth. So, presumptuously sinning is standing against what God stands for (righteousness).

There are other unintentional sins a person can commit that still need repentance and forgiveness but that do not lead to death (1 John 5:16). They don't come out of a rebellious heart but highlight things that need to be ironed out as the person grows in the grace and knowledge of our Lord and Savior Jesus Christ (2 Peter 3:18). For instance, mistakes in judgment happen, thoughts must be guarded, attitudes must be checked, especially with pride that is a blinding agent masking one's flaws and abundant need of God's grace to be transformed according to his will.

See also:

Can You Lose Your Salvation?