You only need to fill out one release form for each individual game you create and sale in UE4. A separate release form is not needed for each individual DLC within one game.
For royalty calculation, you would add all in-app sales for the given quarter, for each game you are selling. For example, if you have one game with 5 different DLC’s within that game, you would add the revenue from all 5 DLC’s to determine total gross revenue for the quarter.
If you’re total gross sales for an individual game, including all DLC sales within that game, is less than $3,000 in a quarter, no royalty is due. Royalty is only calculated on gross sales over $3,000 in any given quarter.