5 post per cat in static page

hello,
i have 3 category in the static page and 15 post per page.
how can i limited the number of posts for each category to 5 in static page?
3*5=15 per page

the code for the category:

function my_home_category( $query ) { if ( $query->is_home() && $query->is_main_query() ) { $query->set( ‘cat’, ‘1’23,42′); } } add_action( ‘pre_get_posts’, ‘my_home_category’ );
endif;

TNX