If you ask a player how many cards are in their hand, do they have to answer you?

The number of cards in each players' hand is public knowledge, so while they don't "have" to answer you, they must make that information accessible to you (by letting you count etc).

And no matter the way they let you know, it has to be accurate (and true). Lying about how many cards you have in hand is cheating. And that way lies DQ.

Misrepresentation
Misrepresentation refers to any attempt to gain competitive advantage through disingenuous or insincere means. Some examples include stalling a game for time, “forgetting” to acknowledge passive effects, acting out of turn, rushing the game state forward without giving an opponent the opportunity to act, intentionally misrepresenting open information (for example, attacking and declaring 10 STR, when the reality is 12 STR), intentionally misrepresenting the known interpretation of a card or rule to a less experienced opponent, misrepresenting a known data point (such as number of cards in hand , amount of power on a card, or the amount of power needed to win), or mis-reporting the results of a game to a tournament judge.