It's not very obvious on the website but you can add multiple cards at once. Say if you study #30-50 today, there is no need to add cards one by one on the Study page.

Simply go to Manage Flashcards > Custom Selection and enter something like 30-50.

If you study in order and don't skip any cards, then it's simpler : go to Manage flashcards. The default is to add cards in order. In which case you'd enter the maximum frame number, so in our example simply enter 50 and all cards up to #50 will be added.

In both cases you get a confirmation step which shows all the cards that will be added.

Adding cards in order is also an easy way to "plug the holes' if you have been skipping some cards you can just enter "50" for example and any missing cards get added - any existing cards are left untouched. No need to add missing cards one by one. This is valid also for any custom range of cards if you use "Custom selection".