All the major scales can be summarised in the following diagram known as the Cycle of Fifths (or the Cycle of Fourths).
If you start at the top of the cycle, C, and go in a clockwise direction, each new key and each new sharp is a fifth higher than the previous key or sharp. The key of F# contains six sharps. After F#, the next logical key would be C# (containing seven sharps). However, this is not practical, and rather than using the key of, C#, you would use the enharmonic name for this key, D# (which contains five flats).