Remember the owner's name in a new private instance field, and call the
"search" method with the owner's name to find the number of borrowers
(this assumes that there are no borrowers with the same name as the real
owner).

import java.util.Stack;

public class Borrow {
    private String itemName;
    private String itemOwner;
    private Stack hasIt = new Stack();

    public Borrow(String name, String owner) {
	itemName = name;
	itemOwner = owner;
	hasIt.push(owner);	// owner's name goes first
    }

    public void borrow(String borrower) {
	hasIt.push(borrower);
    }

    public String currentHolder() {
	return (String)hasIt.peek();
    }

    public String returnIt() {
	String ret = (String)hasIt.pop();
	if (hasIt.empty())	// accidentally popped owner
	    hasIt.push(ret);	// put it back
	return ret;
    }

    public int borrowers() {
	return hasIt.search(itemOwner) - 1;
    }
}
