Best estimate: 140 million

G. Leuba, R. Kraftsik "Changes in volume, surface estimate, three-dimensional shape and total number of neurons of the human primary visual cortex from midgestation until old age" Anat Embryo1 (1994) 190:351-366

Older estimate: 114 million

Klekamp J, Riedel A, Harper C, Kretschmann H-J "Quantitative changes during the postnatal maturation of the human visual cortex." 1991 J Neurol Sci 103/2:136-143

Rough estimates (pooling different sources for volume and density): 205 to 285 million

• average volume of V1 (L and R hemispheres, resp.): 5119 and 5692 mm³
  Timothy J. Andrews, Scott D. Halpern, and Dale Purves, "Correlated Size Variations in Human Visual Cortex, Lateral Geniculate Nucleus, and Optic Tract", The Journal of Neuroscience, April 15, 1997, 17(8):2859-2868
• packing density in area 17: 40,000 cells / mm³
  G. Leuba and L.J. Garey "Comparison of neuronal and glial numerical density in primary and secondary visual cortex of man", Experimental Brain Research, Volume 77, Number 1 / August, 1989
• quoted packing density of neurons in human cortex: 50,000 cells / mm³
  The Quest for Consciousness: A Neurobiological Approach (Koch) - Bottom of Page 71
  
  
• Thus, average number of neurons = (5119 to 5692) * (40000 to 50000) = 204.76 - 284.6 million

an aside: how this compares to number of fibers from the retina

• The average human retina contains 1.07 million ganglion cells
  (inter-individual variability observed: between 700e3 and 1500e3)
  Curcio, C.A. and K.A. Allen 1990 "Topography of ganglion cells in human retina." J. Comp. Neurol. 300:5-25.
• Thus, average number of V1 neurons per fiber in optic nerve:
  • best estimate: 140e6 / 1.07e6 = 131 V1 neurons / retinal fiber
  • low estimate: 114e6 / 1500e3 = 76 V1 neurons / retinal fiber
  • high estimate: 285e6 / 700e3 = 407 V1 neurons / retinal fiber
• Yes, 13100 cortical neurons in V1 per 10x10 patch of fibers from retina! Why?