I think you can make use of the iteration property. To do this I believe your foreach loop needs a name:
Code:
{foreach from=$orders.0.products item=prod name=products}
{$smarty.foreach.products.iteration}
{/foreach}
Another idea is to assign a variable and increment it yourself each time:
Code:
{assign var="increment" value=1}
{foreach from=$orders.0.products item=prod name=products}
Increment: {$increment}
{assign var="increment" value=$increment+1}
{/foreach}