I appreciate you responding.

I would say 250+ and no more than 1000 should be more than enough. Usually doing 20..40 new DAILY is a max if you are doing anything bigger.

I would allow users bigger db (1000..5000) but would limit to the moment they will reveal (open) 250 new cards.

You want them to create habbit. This needs more than few weeks. Completly dont worry about them using 200 or 500, if habbit do not kickin, they will leave you no matter what you offer.

p.s. my experience come from doing 20k rep spacing english. It took me around 10 years of daily doing it.

p.p.s. I paid for my 20k english course, 15 USD. that was for the app, complete 20k cards with phrases and VOICE audio. Mind it. 15 usd. Your tool would cost 10yrs125 whooping 6000 usd :)