What is the longest known sequence of forced moves leading to a checkmate for one side? By forced move, it is meant that the player has only one legal move.

[fen "7k/q5Q1/p4PPK/6PP/8/5P2/8/8 b - - 0 1"]

1... Qxg7+ 2. fxg7+ Kg8 3. f4 a5 4. f5 a4 5. f6 a3 6. f7#


Above is an example with 10 half-moves. Each half-move is the only legal one and the end is a checkmate.
If you don't mind a bushel of promoted Queens, you can get 14 half-moves . . .

[Title "14 forced half-moves to checkmate"]
[Fen "3b3k/qqqqq3/rr4NK/7R/5N1Q/7Q/B6Q/7R b - - 0 0"]

1... Rxg6+ 2. Nxg6+ Rxg6+ 3. Kxg6+ Qh7+ 4. Rxh7+ Qxh7+ 5. Qxh7+ Qxh7+ 6. Qxh7+ Qxh7+ 7. Qxh7+ Qxh7+ 8. Rxh7#