Create a data driven page that is the boundary of your 25 counties.
Set up data driven pages and clip to the Index Layer (ArcGIS Pro)
You can also make a large polygon, Clip out the area where the 25 counties are and it will mask the other.
Now a super super cumbersome way that I had to do years ago was I assigned a County FIPS number to every single asset. (Electric Distribution). The system was in 6 counties. And then set up definition queries in each feature class to only show the needed ones. It works but I would avoid it.