There is no Word field formula to do that for columns containing a mix of numbered cells with empty cells or cells containing text instead of numbers. If you have at least one fully-populated or one completely empty column (insofar as numbers are concerned in either case), you could use a formula like:
{=MAX(COUNT(ABOVE)-1,COUNT(LEFT),COUNT(RIGHT))}
Another workaround might be to insert an empty column (which you can come close to hiding) and count it's rows.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]