The basic 'entry' subscription plan naturally only allows one character per server, to a maximum of 8 characters in total, so if you have more than one character on the same server, you will have to decide which character to keep active (the Mogstation will give you that choice when finalizing the subscription).

Characters that become inactive if you choose an entry level subscription luckily are not deleted (characters are only deleted if you cancel the entire account), hence if at a later date you decide to upgrade your entry level subscription to a full ('standard') subscription, you will be able to reactivate any suspended characters and they will be completely intact will all levels, gear, items etc. So don't worry!