[royng]

I got it. it's simple!

Just use
<form method="POST" action="/store/change_ppp.php" id="pppForm">
Show products per page
<select name="ppp" onchange="this.form.submit()">
<option value="9" {if $products_per_page eq 9}selected{/if}>9</option>
<option value="12" {if $products_per_page eq 12}selected{/if}>12</option>
<option value="15" {if $products_per_page eq 15}selected{/if}>15</option>
<option value="18" {if $products_per_page eq 18}selected{/if}>18</option>
<option value="21" {if $products_per_page eq 21}selected{/if}>21</option>
<option value="24" {if $products_per_page eq 24}selected{/if}>24</option>
<option value="99" {if $products_per_page eq 99}selected{/if}>all</option>
</select>
</form>

in products.tpl