You can manage more planets personally and be able to hire more administrators if you invest into colony management skill, or you can use alpha cores which don't count for administrator limit. If you want to edit files, check out starsector-core/data/config/settings.json. There's also Nexerelin mod which gives you a bit more administrators, when you get mamy enough colonies or when they grow.
Just want to add on that for Mac users (if any), the file is here: Contents/Resources/Java/data/config/settings.json