The Five-hundred-meter Aperture Spherical radio Telescope(FAST) is expected to complete its commissioning in 2019. FAST will soon begin the Commensal Radio Astronomy FasT Survey(CRAFTS), a novel and unprecedented commensal drift scan survey of the entire sky visible from FAST. The goal of CRAFTS is to cover more than 20000 $deg^{2}$ and reach redshift up to about 0.35. We provide empirical measurements of the beam size and sensitivity of FAST across the 1.05 to 1.45 GHz frequency range of the FAST L-band Array of 19-beams(FLAN). Using a simulated HI-galaxy catalogue based on the HI Mass Function(HIMF), we estimate the number of galaxies that CRAFTS may detect. At redshifts below 0.35, over $6, times , 10^{5}$ HI galaxies may be detected. Below the redshift of 0.07, the CRAFTS HIMF will be complete above a mass threshold of $10^{9.5},M_{odot}$. FAST will be able to investigate the environmental and redshift dependence of the HIMF to an unprecedented depth, shedding light onto the missing baryon and missing satellite problems.
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