A moment ago, I enjoyed listening to Pyotr Ilyich Tchaikovsky’s Marche slave, Op. 31 (the Berlin Philharmonic conducted by Herbert von Karajan) on my laptop. The piece is usually known by its French title; in English it might well be called Slavonic March in B-flat minor.
The key of B-flat minor has five flats. Someone who is just beginning to learn to read music may well find keys containing lots of sharps or flats rather daunting. I think I am correct in saying that the composer Wolfgang Amadeus Mozart himself did not write a single work in a key with more than four sharps or flats.
The major keys with five or more sharps/flats are B major, D-flat major, F-sharp major and G-flat major. The keys of C-sharp major and C-flat major are also theoretically possible, but are extremely rarely (if ever) used in practice. For example, Frédéric Chopin’s Fantaisie-impromptu, which is in C-sharp minor, has a middle section that ought to be in the tonic major – except that it is in D-flat major instead. The four sharps of the minor key are cancelled and replaced not by seven sharps but by five flats for the major key.
The minor keys with five or more sharps/flats are G-sharp minor, B-flat minor, D-sharp minor and E-flat minor. Again there are also A-sharp minor and A-flat minor, which exist in theory but are almost never used.