Only add as many new cards as you can keep up with the reviews. A good starting point is around 7 cards for every hour per day that you are prepared to study. After a few weeks, if you have too few/many reviews, adjust the amount of new cards until you have a manageable number of reviews every day.

Mathematically, it will depend on the percentage of cards you get right.  If you consistently answer 80% correct and add the same amount of cards each day, after 60 days, your workload will stabilize around 20 times the amount you add every day.  So if you add 10 cards every day and get 80% correct, you can plan on answering approximately 200 cards every day.

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