In almost 40 years, twice.
Never as a player though 8(
The First and still ongoing campaign started in 1983 and has been going on as a "Long Distance Relationship" since. We get together about 4 times a year for a long weekend to continue. Currently the group is sitting at 20-23.
The Second was a freaking freight train.... took off running and in 2 years playing 3 times a week reached 19-20 and then Life kicked us all in the nether regions. Jobs, Kids, Women, Boys, yadda yadda yadda... you get the idea.
I play in a group now that the other GM runs APs exclusively and when they end, the campaign ends 8( It's one of the reasons I have a hard time getting attached to the NPCs/Story Lines as they are all on a spiral the to Bin of Games Past. YMMV
More APs that run to 20 Please!