  The Five-hundred-meter Aperture Spherical radio Telescope (FAST) completed
its commissioning and began the Commensal Radio Astronomy FasT Survey (CRAFTS),
a multi-year survey to cover 60% of the sky, in 2020. We present predictions
for the number of radio-flaring ultracool dwarfs (UCDs) that are likely to be
detected by CRAFTS. Based on the observed flaring UCDs from a number of
unbiased, targeted radio surveys in the literature, we derive a detection rate
of $\ge$3%. Assuming a flat radio spectrum $\nu L _{\nu}\propto \nu^{\beta+1} $
with $\beta$ = -1.0 for UCD flares, we construct a flare luminosity function $d
N/d L \propto L^{-1.96 \pm 0.45}$ (here $L=\nu L_\nu$). CRAFTS is found to be
sensitive enough for flares from UCDs up to $\sim$180 pc. Considering the
Galactic thin disk, we carry out a 3D Monte Carlo simulation of the UCD
population, which is then fed to mock CRAFTS observations. We estimate that
$\sim$170 flaring UCDs would be detected through transient searches in circular
polarization. Though only marginally sensitive to the scale height of UCDs, the
results are very sensitive to the assumed spectral index $\beta$. For $\beta$
from 0 to -2.5, the number of expected detections increases dramatically from
$\sim$20 to $\sim$3460. We also contemplate the strategies for following up
candidates of flaring UCDs, and discuss the implications of survey results for
improving our knowledge of UCD behavior at $L$ band and dynamos.
