Great, thanks for reporting back @yiwen!
Did you observe millions of representative sequences when using the Moving Pictures tutorial data set, or were you following the tutorial's steps using your own data?
Using the Moving Picture tutorial data set with DADA2, we're observing 759 representative sequences (i.e. 759 features) using QIIME 2 2017.12. I just want to make sure you're not observing a larger number of representative sequences if you were using the tutorial data set. Thanks!