What notes are flat in this key?

It depends on which key you are referring to, all the keys with flats are:

F major (1 flat)

B-flat major (2 flats)

E-flat major (3 flats)

A-flat major (4 flats)

D-flat major (5 flats)

G-flat major (6 flats)

C-flat major (7 flats)

Likewise the relative minors are:

D minor (1 flat)

G minor (2 flats)

C minor (3 flats)

F minor (4 flats)

B-flat minor (5 flats)

E-flat minor (6 flats)

A-flat minor (7 flats)