There are or should be at least (bound by national security related laws) oversight review committees and stats reporting...
... but of course it depends on the standard for what 'prevented' means. De-radicalized a potential terrorist, does that mean a terror attack was stopped/prevented.
I mean, aren't they quite busy radicalising the mentally ill just so they can then capture the mark in the process of following the plan they developed for them just so they bump up their 'terrorist plan foiled' stats?
... but of course it depends on the standard for what 'prevented' means. De-radicalized a potential terrorist, does that mean a terror attack was stopped/prevented.
It's quite complicated in practice, alas.