Assuming you mean F major, then the answer is one flat.
There are no flats in Em, just an F sharp!
The F minor scale has four flats.
One, Bb. 1, the flat is B flat.
There can be a total of seven flats (or sharps) in a key signature.
One, Bb. 1, the flat is B flat.
1, Bb.
"Fm" is shorthand for the key of F minor, a key signature with four flats.
When adding flats to a key, the order is B, E, A, D, G, C, and finally F. With sharps, it's the exact opposite.
F major, B-flat major.
None, only flats. B flat in F major, B, E, A, and D flat in F minor.
F# minor has a key signature of 3 sharps, whereas Gb minor has a key signature with 6 flats. It's easier to use key signatures with fewer flats/sharps.
Bb C Db Eb F Gb A Bb Four notes have flats. The key signature would contain five flats for Bb minor, with accidentals used on the A naturals.