On Mahashivaratri, special Shiva is also called Sahasramukhi i.e. thousand mouths, but mainly Shiva is Panchmukhi. Shiva runs the world through these five heads. Shiva's favorite number is five. Shiva is centrist . That is, neither of the devas nor of the asuras. He is in the middle, he can get whatever he wants. When zero exploded, nine numbers came out of it - one to nine.
Five is the middle number. Shiva's favorite mantra is Namo Shivaaya. It also has only five letters, that is why it is called 'Panchakshara Mantra'. Shiva is the deity of the five elements. The senses are also five and Shiva is also the master of the senses. According to numerology, if someone's fortune digit is five, then he should worship Shiva specially. Shiva is quickly pleased at that.