@Juztsacki: Your question is pretty simple, yet can mean a lot of things to me.
1) If you mean "can you have only 1 party member visible on the map irregardless of party members", then you can go to the Database, select the System tab, and untick the 'Show Player Followers' option.
2) If you mean "can you have only 1 party member in you party", then you can do one of the following depending on the situation:
- For a starting party, go to Database and select the System tab. Make sure to add only one actor in the Starting Party.
- For a starting party or when changing out party members, use the event command 'Change Party Member' to add and remove them.
3) If you mean to change the UI to suit a one actor party in both menu and battle scenes, you need custom scripts for them, and can check the JS plug-in section if you can find one you can use. Alternatively you can request or commission someone for it, because this will take more than a few minutes effort to achieve and would usually be specialized in appearance and functionality depending on the individual's needs.
4) If you mean to have only 1 party member participate in battle irregardless of number of party members in your roster, then you can make a quick script edit to change that, like what Andar mentioned. DreamX's script looks like a good option as well.
5) If you mean to have followers who 'follow' the party but don't appear in the roster (like an NPC follower), you can try DreamX's script as well. His script looks very well done and will be able to produce a result you might need.