He has a huge amount of vocabulary.

[Silverobama]

If someone knows many English words, is it natural to say "He has a huge amount of vocabulary".

 

[Rover_KE]

It’s not natural to me. I’d say ‘He has a huge vocabulary’.

 

[Skrej]

Of course, using Rover's pattern, you could also substitute many different size-related adjectives.

big, huge, extensive, expansive, small, minuscule, limited, tiny, etc.