Let me breakdown:
Re: (1L = 1000 cm^3)
water has an approximate density of 1g/cm^3 (temperature dependent)
Therefore, 1000 cm of pure water has an approximate mass of 1000g
Mass of 1 mole of water = 18.02 g
Hence, no. of moles water in 1000 cm^3 of water = Total mass/mass of 1 mole = 1000...