Magic Tricks

Your assumption is correct...you remove the white inserts.
Although I don't use any packet tricks that requires "up to 20 cards", you can fit 20 cards (10 per side) into the wallet easily.
Note: these wallets are very well made and are much better than the plastic varieties available.
In fact, it's the only 'packet' that I use now.
Now the problem...availability.
PM runs out of these things all the time.
As you can see, as of this typing, the wallets are 'out of stock'.