There is not a exact number to train a classifier. You can have a large dataset where all the data has the same attributes so you classifier will memorize a pattern, or, you can have a no so big dataset with good instances so you classifier will have better results.
You can train the classifier using the sample dataset that they give you in the post and use the cross validation in order to get the best classifier.
After you got the best classifier, you can compare your classifier with the classifier provided in the post and choose the better.