In Jain Dharma, “9 Ang” usually refers to the 9 Ang Agams that are specially accepted in the Shwetambar tradition as part of the original Jain canon.

Why 9 Ang?

Because the Ang Agams are the principal limbs of the Jain scriptures, and in the present Shwetambar scriptural tradition, 9 Angs are recognized as preserved and available.

They are:

1. Acharanga
2. Sutrakritanga
3. Sthananga
4. Samavayanga
5. Bhagavati
6. Jnatadharmakatha
7. Upasakadasa
8. Antakritdasha
9. Anuttaropapatikdasha

Important point

Traditionally, there were 12 Angs in Jain scripture. But in the current age, the last three Angs are considered lost in their complete form, so only 9 Angs remain recognized in full availability.

In simple words

9 Angs are accepted because they are the currently preserved core Ang scriptures of Jain canon, especially in the Shwetambar tradition.

If you want, I can also explain why there are 12 Angs in Jainism and which 3 are missing.