by Ernie » Tue Oct 09, 2018 6:51 pm
9000 cards should be no problem. I've tested the app with 20,000+ cards is it's fine. But 400 decks sounds difficult to navigate and maintain. You might consider combining them and using "Category 1" column to assign a category or tags which can allow you to filter them for studying. For example, my cards have a category for book+chapter, so I can turn individual chapters on/off.
You mentioned sync. The normal process to get cards in and out of the app is to transfer using my website, Dropbox or Google Drive. You can import/export and also run the "download/update" process on an existing deck to redownload cards from cloud (which will replace all cards, but the app will try to match up old and new cards so you don't lose your statistics).
I have a "sync" feature, but it only works with Dropbox and is not fully automatic. The backup to Dropbox is automatic, but the downloading from cloud is not as you need to help it along. This sync feature is designed to help keep two different devices in sync (if using 2+ devices) or as an automatic backup. It's not a particularly efficient process. If you have 200 decks, and you study one card from each, the app will need to export or import all cards from the 200 decks, or export/import the complete database file of all cards each time.
I'm currently working on an improved sync that will be much more efficient and be fully automatic.