Sadly, this isn't something that yo ucan change in the templates as far as I am aware. You might be able to dig into the kernel and make a change there, but I would recommend against it.
Alex
[ bald_technologist on the IRC channel (irc.freenode.net): #eZpublish ]
you need to alter the file kernel/classes/ezrssexportitem.php. At line 224 you will find the initalization of the number of items, $num = 5, change this number to e.g. 10 and you're done.