How to assure the best cluster number for the given instances?
No, you can't exclude that there are some clusters in this data.

Maybe they:
- can not scored by CH and Silhouette
- can not be found by k-means (but by other algorithms)
- require better preprocessing
- were destroyed by the preprocessing you applied
- exist only in subspaces

But I would agree that none of the k-means results results you got was good on Silhouette. Which is not surprising given 128 variables.