Does menace require exactly two blockers or at least two blockers?

Menace requires that the creature with menace be blocked by at least two creatures, not exactly two. If a player wants to block a creature with menace, they must assign two or more blockers to it. Blocking with only one creature is illegal.

This is defined in CR 702.111b: a creature with menace can't be blocked except by two or more creatures. The rule sets a floor of two, not a ceiling. Three, four, or even more creatures may all block the same menace creature if the defending player chooses.

This matters practically because a player might want to 'chump block' a large menace creature with three smaller creatures to prevent all damage to their life total, and the rules fully allow that.

Example: Your opponent attacks with a 5/5 with menace. You have a 1/1, a 2/2, and a 3/3 available. You may legally block with all three of them — or with any two of them — but you cannot block with only one. Attempting to assign only the 3/3 as a blocker would be an illegal block and the declare blockers step would need to be corrected.

Remember that even if two or more creatures block a menace creature, the attacking player still chooses how to order and assign combat damage among the blockers (CR 510.1c), so simply meeting the two-blocker minimum doesn't guarantee safety for all blocking creatures.