We present confusion-limited SCUBA-2 450 μm observations in the COSMOS-CANDELS region as part of the James Clerk Maxwell Telescope Large program SCUBA-2 Ultra Deep Imaging EAO Survey. Our maps at 450 and 850 μm cover an area of 450 arcmin². We achieved instrumental noise levels of σ₄₅₀ = 0.59 mJy beam⁻¹ and σ₈₅₀ = 0.09 mJy beam⁻¹ in the deepest area of each map. The corresponding confusion noise levels are estimated to be 0.65 and 0.36 mJy beam⁻¹. Above the 4σ (3.5σ) threshold, we detected 360 (479) sources at 450 μm and 237 (314) sources at 850 μm. We derive the deepest blank-field number counts at 450 μm, covering the flux-density range of 2–43 mJy. These are in agreement with other SCUBA-2 blank-field and lensing-cluster observations but are lower than various model counts. We compare the counts with those in other fields and find that the field-to-field variance observed at 450 μm at the