The number of keys to meet duelists is based on their level. Players have to use the more number of keys as the duelist's level is higher.
Required Key QTY (Neutral)
Required Key QTY (Color)
How to Get
Players can obtain gate keys as rewards for winning duels. Having duels against standard duelists will be the main way to collect the items. Standard duelists take 30 minutes to reappear after defeated, and players can get them to show up immediately by using the Duel Orb.
Gate keys are also rewards for completing weekly and stage missions.
Players can gain the items by defeating other players, although they will be stronger than standard duelists.
You can also convert no-colored gate key into the colored one by meeting Card Trader. However this way is undesirable relative to other ways noted above, since so many neutral gate keys are required to have a duel against legend duelists.
Need more keys? Then just try playing Casual Duel, and you can get them fairly quickly from all of the disconnect "losses" that occur even before the duels have started. Yes, you can even do this with a good internet connection. For some reason, it happens a lot more in casual than in ranked. (More people, I guess). And another key farming tip: Don't surrender in pvp unless you have to. If you lose, you can still get keys, but if you surrender, you get NOTHING.