How many neurons are there in the human spinal cord?

A: approximately 18.5 million

43,000 neurons per millimeter spinal cord length

(Burish et al 2010, Brain Behav Evol 2010;76:45–59 https://doi.org/10.1159/000319019)

377 - 481 mm long
(Leo et al 2014, Spine. 39(4):E262-E269, doi: 10.1097/BRS.0000000000000125)

Total number of neurons: 43,000 x ~429 = 18,447,000