music sheet maker

No, the normal way is to have exactly as much parts as you setup on score creation. But you and add or delete them later too.

To be clear: go to File / New to create your score and choose exactly as many instruments and staves as you want. Not sure where you're seeing two parts, though, the default empty score has only one. So presumably you have already used File / New and chosen two instruments or staves? Next time just choose one. But you can also delete one from your current score using Edit / Instruments.