Are you required to inform your opponent how many cards you have sideboarded?

No, you are not required to inform your opponent how many cards you have sideboarded.

According to Rule 403.6 of the Tournament Rules, "Players may count the cards in their opponents’ sideboards at any time." Because the sideboard (and the number of cards within it) is considered public information, your opponent is permitted to count them to determine if and how many cards you have changed, but you are not required to provide that information to them yourself.

Additionally, per Rule 501.3, players are generally not required to assist their opponents in playing the game.