Upon clicking any of your cards within your collection, you may notice a column defining a card's BCX, or "base card experience", located next to any cards' "Lvl" (level). BCX represents the total number of this copy of a card that was combined into one card, which eventually affects a card's level once enough copies are combined.
Upon clicking on the "stats" tab within the same page displaying the BCX of the card, you will see a list of levels that this card is able to achieve. To the right of the level column, you will see a column that displays the number of cards needed to be combined to achieve the listed level for the card (circled in green). If you want to level this card up to level 4, then 30 copies are needed. If you want to level this card up to level 5, then a total of 60 copies are needed instead.