I'm not able to see more than 6 thumbnails on the index page.
I'm able to see more than 6 thumbnails on the gallery page.

I have gallery settings as sort by ID.
And Albums per page set as 12.
And im not using any caching plugin
And i have published all the catergories.

I'm using stop design theme.

Please could someone help me.

That would be because that is the design of the stopdesign theme.