Computed: chai (חי) = 8+10 = 18. Counted: the eighteen benedictions of the Amidah.
Meaning
Eighteen is the number of life, and the tradition has made it the most ordinary sacred number it has: charity is given in multiples of eighteen, a gift is an eighteen, and the daily prayer is the Shemoneh Esreh, the Eighteen. The coincidence of the prayer's count and the word for life is one the tradition enjoys without offering it as the prayer's reason.