Counting is a means of regulating one’s chanting, according to Srila Bhakti Vinoda Thakura in Bhajana Pranali.
Counting japa on beads is the thirty-fourth limb of bhakti as described in Jaiva Dharma and found in NOD.
The reason for chanting upon beads or counting upon one’s fingers is to ensure reaching a fixed number of krsna-nama and to note whether one’s chanting is increasing or not. Tulasi devi is very dear to the Supreme Lord, therefore chanting harinama in contact with her in the form of tulasi beads enhances one’s feeling and attachment for harinama. (from Jaiva Dharma by BVT)
In the early morning, after completing his dictation of Srimad-Bhagavatam translations at maybe 4 or 5 A.M., Srila Prabhupada would sit back and chant japa. He did not chant very loudly, but from the other room one could sometimes hear, especially the words, “Rama, Rama.” Then in the afternoon sometimes he would remark that he had a few extra rounds to chant, and he would move the counting beads. We took it that Prabhupada’s statements about his own chanting of prescribed rounds were for our benefit, but it was encouraging for our own regulative habit. (Prabhupada Nectar, SDG)
Hari Bhakti Vilasa states:
After becoming a devotee or engaging in the devotional service to the Supreme Lord, one who does not accept a japa mala, his initiation is said to be demoniac. This kind of initiation is not very beneficial for religious principles. One who chants on the beads properly, I (the Supreme Personality of Godhead) think of this intelligent person for thousands of years.
The beads which have 108 seeds (beads) are said to be of he topmost quality. The beads which have 50 seeds are said to be of medium quality. The beads which have 20 seeds are said to be of the lowest quality.
One should understand that beads made out of Tulasi wood seeds is said to be fulfilling all kinds of desires in every activity or function.
The mala which is made out of Tulasi wood seeds is said to be certainly rewarding liberation.
One who chants japa holding the beads on the tip of the fingers, who jumps over the head bead or who chants japa without counting how much he has chanted, then that japa is said to be fruitless.
If one chants japa regularly to please the Supreme Lord, then the Supreme Lord, being pleased, will increase one’s spiritual advancement and give one the eternal liberation.
One should chant regularly, according to one’s capacity, a fixed number of rounds every day.
Srila Sanatana Gosvami says in his Digdarsini-tika, that according to one’s capacity, one should regularly chant a fixed number of rounds each day and account the number each day. “One should not chant less or more japa day after day”. When the fixed number of rounds is completed one should finish chanting japa and engage in other service.
That japa is fruitless unless one counts his rounds.