Adding "x" bars rest to a score

[Ctrl]BBBBBBBBB will add 9 empty bars to the end of your score.

Or perhaps you are asking about multi-measure rests. If so, things to note:

For a multi-instrument score you normally do not use multi-measure rests. They are shown only when there are no instruments playing and there is normally someone playing something at all times.

Multi-measure rests are used in parts extracted from a score where a particular instrument is not playing.

By default musescore does not show multimeasure rests in a score but does show them in a part. Whether viewing a score or a part you can toggle multimeasure rest display on and off with keyboard shortcut M.

Thus, to have a 9 bar multi-measure rest just leave 9 empty bars and then if necessary press M.

See https://musescore.org/en/handbook/4/measure-rests-and-multimeasure-rests