The answer is 6.
Except for the S productions, $S \rightarrow SS | ab$, all other are useless since we cannot reach them. So we can simply ignore them.
Now, the string $ababab$ have two derivation trees, hence is ambiguous.
$S \rightarrow SS \rightarrow abS \rightarrow abSS \rightarrow ababS \rightarrow ababab$
$S \rightarrow SS \rightarrow Sab \rightarrow SSab \rightarrow Sabab \rightarrow ababab$
I have not shown the derivation trees. But these derivations will give you the idea.
In the first derivation, we expand the second $S$ to get $SS$, while in the second derivation, we expand first $S$ to get $SS$. From this idea, I think you will be able to come up with two different derivation trees for $ababab$.
Any string shorter than this won't work. Why?
These are the derivable strings with the length less than 6: $ab$, $abab$.
Both of these strings have only one derivation tree.
