How many mgs of citalopram does it take to overdose?

No specific number: The general maximum dose for citalopram is 80mg though sometimes a higher dose may be used in ocd. Generally speaking it is very difficult to overdose on ssris, and people can tolerate hundreds of miliigrams of citalopram without dying.

"Overdose" vs lethal: "overdose" means taking more medication than prescribed or intended. The amount you take may or may not be lethal, which differs by medication and your tolerance. I agree with dr. Tripp that citalopram and other ssri's have a high safety record in overdose. A variable amount may give you unpleasant -- but probably not lethal -- side effects.