Inside of the class that eats the leaves(I'm guessing that you are doing the Wombats tutorial), at the top of the class, add a variable and call it whatever you want, like this:
public class eaterOfLeaves extends Actor
{
//you can put your variable to keep track of leaves eaten(it would be an instance variable) right below:
private int numOfLeavesEaten
//the rest of your class
}
Then, when you want to eat a leaf, also update the numOfLeavesEaten, by typing numOfLeavesEaten++. When you want to check how many have been eaten, just get the value of numOfLeavesEaten.