There is a function to limit the number of items pulled per feed: set_item_limit()
$feed->set_item_limit(5);
However, this pulls the same number of articles from each feed. You could set that to 10 or whatever you want your max to be. The only way I can think of (off the top of my head) to do what you want is to set your own counters in the get_items loop and each time through check something in the feed data like the url or something.
$cntr1 = 0;
$cntr2 = 0;
foreach ($feed->get_items() as $item) {
if ($cntr1 <= 5 && strstr($item->get_feed()->get_permalink(), [your search criteria])) {
// do your fetch of the item stuff for feed 1
$cntr1++;
}
...
}