In this paradigm, Light Noon does not come after an Alif.
That excludes exactly 6 positions:
• all 4 dual forms
• both 2 feminine plural forms
So:
4 + 2 = 6
The remaining 8 positions are the forms in which Light Noon can occur.
Opening interactive controls...
Why Light Noon Is Excluded from Six Forms
Structural Rules of Heavy and Light Noon-e-Ta'keed
In this paradigm, Light Noon does not come after an Alif.
That excludes exactly 6 positions:
• all 4 dual forms
• both 2 feminine plural forms
So:
4 + 2 = 6
The remaining 8 positions are the forms in which Light Noon can occur.