YMMV. In Prince Charming, Reanimator, I have seen near TPKs and parties survive with most of their members intact.
In AG, there are a few areas of maximum potential carnage - if you have read the module, you know which they are. The highest point, the lowest point, and the sacrifice. As befits a Lovecraftian story, though, much of what could potentially happen relies upon the curiosity of the PCs. There are at least three encounters, played poorly, that can result in a TPK. If they engage in the PVP option near the start of the scenario, half the party can be gone before they open the first door.
Even if the entire party of 16 survives (unlikely as that may be), this just means that the players have more than one potential PC to choose from. This can be good or bad. I ran Well of the Worm as a funnel, and one player consistently got another player to let her characters lead the way. At the end of the day, the first player had 3 PCs ready to level, and the second player was wiped out.
i like to try to flash out a little bit funnel-fodder when i play but it seems to me that there are players who are more utilitarian in their approach.