NumberAxis::SetLabelCount doesn't work unless UpdateTickValues() is called after updating the value of m_labelCount. In the released version the only way to do this (as far as I can see) is to call either NumberAxis::UpdateBounds or NumberAxis::SetFixedBounds after changing the label count. I fixed this problem for myself by editing numberaxis.h and changing the definition of the SetLabelCount function like this:
void SetLabelCount\(size\_t labelCount\)
\{
if \(m\_labelCount \!= labelCount\) \{
m\_labelCount = labelCount;
//-----------------------
//added this line to fix bug
UpdateTickValues\(\);
//-----------------------
FireAxisChanged\(\);
\}
\}