Set posts per page for tag link

I use the get_the_tag_list to get links to my tags.
now to have my customized way of showing the posts
from that tag that was selected, i changed the tag.php file
to my own structure inside a loop that starts like a regular loop
while ( have_posts() ) : the_post();

so then wordpress somehow (i don’t really know how it’s working)
knows to get only this tag posts to the loop.

the problem is I wan’t to set the number of posts per page
specifically for the tag.php and not from the wordpress setting
global option to set posts per page.

the way I do that in another page is:

$paged = (get_query_var('paged')) ? get_query_var('paged') : 1;
query_posts('cat=9&posts_per_page=8&paged='.$paged);

but I can’t use it here because then it dosen’t know to get the selcted
tag posts only…

Any Idea ? Thank’s!