Here's why it's difficult to determine:
* Ambiguity: The story focuses on Alice's interactions, not a complete inventory of every character.
* Constantly Changing: The Mad Hatter and March Hare are known for their unpredictable behavior, suggesting a party with a fluid, constantly changing guest list.
* Fantasy Element: The story is a fantastical world where logic doesn't always apply. There could be more characters than initially appear, or even characters that disappear and reappear.
While we can identify some key attendees like:
* The Mad Hatter
* March Hare
* Dormouse
* Alice
* The White Rabbit
The exact number of characters at the Mad Tea Party remains open to interpretation.