Display post different after specific number of posts?

either add a counter to the loop, and use a conditonal statement;
or use the $wp_query->current_post variable with a conditional statement.

for ‘only on index page’ look into http://codex.wordpress.org/Conditional_Tags

is it possible to get som example code?

please

tis will depend on the level of ‘style/display posts different’ and on your existing code.

very general example:

<?php if( $wp_query->current_post < 9 ) : ?>
show post in one way
<?php else : ?>
show posts in a differnt way
<?php endif; ?>

this could be as simple just within the post div to set a css class for the two groups of posts, or wrapping the full output section so you can have a total diffrent html and oputput structure.

needs to be within the loop.