NTerms[x] is equivalent to Length if x is a sum; otherwise NTerms[x] returns 1, except NTerms[0] -> 0.
NTerms[x]
Length
x
1
NTerms[0] -> 0
Overview
NTerms[a - b]
2
NTerms[a b c]
NTerms[9]
NTerms[0]
0