Music is built on the number 7. There are 7 sharp keys, 7 flat keys, 7 numbers, 7 letters, 7 common dynamics and so on…….It’s very mathematical however it’s not hard to understand the basic concepts if you remember some very simple steps while you approach sight-reading.
Here is one concept regarding key signatures which will help you.
Every letter in the sharp keys has an opposite letter on the flat side. The addition of these two numbers equals 7. For instance, if you notice how the key of F has 1 flat. The key of F# has 6 sharps. Added together that equals 7.
If you know that the key of A has 3 sharps, then what plus 3 equals 7? 4. So the opposite key of A which is Ab must have 4 flats.
E has 4 sharps, so Eb has 3 flats and so on.