Staves per [age

Since you have a solo instrument, each 'system' is comprised of a single staff...
Use menu item: Format > Style > Page, and select 'Disable vertical justification of staves'.
Then you can adjust the minimum and maximum system distances to fit more single line staves per page.
Just make sure you do not have any explicit page breaks set. You want the single line staves to be able to flow freely from one page to another as adjustments are made. Explicit page breaks will prevent this.

Actually, it would not normally be a good idea to reduce the minimum system distance - it will make the music look unprofessionally crowded, and it will often be ineffective anyhow, since MuseScore may need to add space to avoid collisions. It's almost always better to reduce the staff size if you need to fit more music - this creates a less crowded result. See Format / Page Settings / Staff Space. And if you need further assistance, just attach your score so we can understand and assist better.