How to Write Multiple Instruments for big orchestra or ensemble

https://musescore.org/en/handbook/create-new-score#instruments-and-voic…
Choose how many instruments you want, use the central buttons to order them;
Adjusts page settings https://musescore.org/en/handbook/page-settings if necessary

The bottom line is that you can add as many duplicate instruments as you like. If the instruments have the same dynamics and basic rhythm then you can add non-linked staves ( this is ugly - i.e. may not work) or just add duplicate instruments until you have all the instruments you want. The "I" button brings up the same instrument menu you started with when you made you score with a non template list of instruments. It works the same as if you brought it up with no instruments.