How many pebbles were in Stargirl's wagon?

Depending upon where you are in the text, there are different numbers of pebbles in the wagon. At times, there are only 2 pebbles in the wagon. The most that are reported are when Leo comes to dinner. At that point Stargirl has 17, and she puts in another one because his coming to her house has made her happy (making 18 total).