I'm scared, for I've been wondering whether the Pharisees commit the unforgivable sin by just once saying blasphemous things about the Holy Spirit? I know it's said that the unforgivable sin isn't any single act from which there's no more turning back, but how does it seem that only a few words from the Pharisees made them fall guilty of the unforgivable sin? I'm confused.
You are confused because you are making a number of assumptions that lead to conflicting ideas.
After Jesus healed a demon-possessed man who was unable to speak or hear, the crowd was beginning to realize that Jesus just might be the Messiah. In response, the Pharisees launched a counter rumor: "This man casts out demons only by Beelzebul the ruler of the demons" (Matthew 12:24). This isn't the first time they spread this idea. Some scribes from Jerusalem where spreading the same rumor earlier (Mark 3:20-30; Matthew 9:34). Just because you see a rebuke for a statement, you cannot conclude that this is the only time it has happened. The gospel records show that it happened at least twice.
In both cases, Jesus does not say the Pharisees would never be forgiven. He states that blasphemy against the Holy Spirit would lead a person to a state from which he won't seek forgiveness. We don't know if these Pharisees had reached that state or not. (See: Blasphemy Against the Holy Ghost.)
Slandering the Holy Spirit to get people not to believe in God (i.e. blasphemy) isn't the only sin that leaves a person unable to be forgiven. "If anyone sees his brother sinning a sin which does not lead to death, he will ask, and He will give him life for those who commit sin not leading to death. There is sin leading to death. I do not say that he should pray about that. All unrighteousness is sin, and there is sin not leading to death" (I John 5:16-17). Any sin which a person commits and refuses to repent of is a sin that leads to death because repentance is a prerequisite to forgiveness. (See: A Sin Leading to Death.) What is true is that a person willing to slander the Holy Spirit to spread unbelief is very unlikely to repent of this particular sin.
All right. Thank you.