A tip to select multiple mullions or multiple curtain panels in a curtain wall or curtain system:
MULLIONS
In any view, select the mullion or panel of your choice.
Right click.
There is a command for "Select Mullions".
Select the appropriate option:
CURTAIN PANELS
In any view, select the panel of your choice.
Right click.
There is a command for "Select Panels".
Select the appropriate option: