  We present the deepest 850 $\mu$m map of the SSA22 field to date, utilizing a
combination of new and archival observations taken with SCUBA-2, mounted at the
James Clerk Maxwell Telescope (JCMT). The mapped area covers an effective
region of approximately 0.34 deg$^2$, achieving a boundary sensitivity of 2 mJy
beam$^{-1}$, with the deepest central coverage reaching a depth of
$\sigma_\text{rms}$ $\sim$ 0.79 mJy beam$^{-1}$, the confusion noise is
estimated to be $\sim$ 0.43 mJy beam$^{-1}$. A catalogue of 850 $\mu$m sources
in the SSA22 field is generated, identifying 390 sources with single-to-noise
ratios above 3.5, out of which 92 sources exceed 5$\sigma$. The derived
intrinsic number counts at 850 $\mu$m are found to be in excellent agreement
with published surveys. Interestingly, the SSA22 number counts also exhibit an
upturn in the brighter flux region, likely attributed to local emitters or
lensing objects within the field. On the scale of $\sim$ 0.3 deg$^2$, the 850
$\mu$m number counts are unaffected by cosmic variance and align with the blank
field. In the deep region ($\sigma_\text{rms}$ $\leqslant$ 1 mJy), the counts
for fluxes below 8 mJy are consistent with the blank field, and the excess in
the brighter regime is not significant. Due to the limited number of very
bright sources and the insubstantial cosmic variance in our field, we attribute
the fluctuations in the number counts primarily to Poisson noise. The SCUBA-2
850 $\mu$m detection in the SSA22 field does not exhibit indications of
overdensity.
