According to Kiranico, the chances of a Plate dropping range from 1% for Caravan quest rewards, up to 5% for a High Rank Horn Break or Tail Carve.

Carve rewards and shiny drops are fairly straightforward; you have an X% chance of getting that item from the drop source.

The reward box is where it gets interesting. You are guaranteed a single item, rolled from the quest rewards table for that quest, and then a roll is made to see if you get another reward, which is affected by (Ultra) Lucky/Unlucky Cat, and the Fate Armor skills. If that succeeds, you are then given another item, rolled from the available quest rewards. This repeats until you fill up all the item slots in the box, or the roll fails to give you another one.

You are also guaranteed one item for breaking parts that give items, like the Shagaru Magala's Horn Break, but they have their own reward pool, for that specific break. So for Low Rank, breaking its horns gives you a 3% chance to get a Plate in the Reward Box.

So to maximize your chances of getting a Plate, you want to fight the High Rank S. Magala, break its horn, cut off its tail, and hopefully you have either the Great Luck armor skill, or Ultra Lucky Cat.

According to Kiranico, the chances of a Plate dropping range from 1% for Caravan quest rewards, up to 5% for a High Rank Horn Break or Tail Carve.

Carve rewards and shiny drops are fairly straightforward; you have an X% chance of getting that item from the drop source.

The reward box is where it gets interesting. You are guaranteed a single item, rolled from the quest rewards table for that quest, and then a roll is made to see if you get another reward, which is affected by (Ultra) Lucky/Unlucky Cat, and the Fate Armor skills. If that succeeds, you are then given another item, rolled from the available quest rewards. This repeats until you fill up all the item slots in the box, or the roll fails to give you another one. For the Plate, the Caravan quest has a 1% chance of a reward being a Plate, and the High Rank Guild quest has a 3%. Note that this is for every roll; every item has a chance to be a Plate, and the possibility of having a full box of Plates exists, although it is vanishingly unlikely.

You are also guaranteed one item for breaking parts that give items, like the Shagaru Magala's Horn Break, but they have their own reward pool, for that specific break. So for Low Rank, breaking its horns gives you a 3% chance to get a Plate in the Reward Box.

So to maximize your chances of getting a Plate, you want to fight the High Rank S. Magala, break its horn, cut off its tail, and hopefully you have either the Great Luck armor skill, or Ultra Lucky Cat.