You can put $posts = 1; in your stub file (if you're using stub files) for blog #1. If you're not using stub files you can try adding this bit to the top of your skin's _main.php file:

<?php 
if( $blog == 1 ) {
$posts = 1;
} ?>

I do the first way so I know it works. I think the second will too.

Thanks EdB,
I don't use stub files...

My home page is index.php , it's the default blog and it's number is 2

I used your code , changing it to 2 but to no avail !!