Guilmon

Q:Does "If the total number of cards in both players' trashes is 20 or more" mean 20 or more cards in either me or my opponent's trashes?

A:No, it means when the cards in your trash and the cards in your opponent's trash add up to 20 or more cards in total.For example, the condition is met if you have 12 cards in your trash and your opponent has 8 cards in their trash.

Q:After gaining from this card's [On Play] effect, is lost if the total cards in both players' trashes is reduced to 19 or fewer cards?

A:No, "for the turn" effects persist as long as the conditions were met at the time of activation.