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They also said that the higher the lvl of the monster the more beneficial it will be.
So you will most probably need lvl 10 commons, which are 400 cards x 5 monster = 2000 cards.
Yaba said in the town hall that there are not enough cards to make every land perfect.

2000 commons as an upper bound is nothing. There's like 8 million of each of the recent reward commons, 125k of their gold foils, millions of each rare, etc. and that's before even considering other sets like CL/RW or the hundreds of millions of other cards that will inevitably be printed in the future. Having only 5 slots per land would be completely absurd. I need a reason to buy another 500k cards, not a reason to start dumping them.