Ok since a variable can let you store a number, here's what you have to do.
If you want the quest to be completed when the hero HAS 10 Bat Wings in his inventory, set a condition that will finish the quest when there is 10 Bat Wings in the hero's inventory.
If you want the quest to be completed when the hero COLLECTS 10 Bat Wings, add +1 to a variable of your choice when the hero collects a Bat Wing. Then set a condition that only activates when that variable is equal or greater to 10.
Unfortunately I don't have RM installed on this computer so I can't get you an actual screenshot.
Basically you need to use a variable to store data (in this case the number of an item) and use a condition. It should look like something like this, I guess.
<>IF there are at least 10 Bat Wings in the inventory,
<>THEN complete the quest
<>ELSE "Old man: You need more Bat Wings! Stop fooling around and go kill those Bats already!")
<>END