What is the treatment after cardioversion procedure for atrial flutter

Yes, at least for a while, and perhaps for the rest of your life. The reason is that an SVT/tachyarrhythmia raises your risk of a stroke. There's truthfully still a lot of discussion about this, and I just saw a new study suggesting that the medical establishment needs to revise downward their assignment of a risk factor for those whose arrhythmias are paroxysmal AND that last less than 3 hours at a time. For the time being, the protocol is to ask patients with arrhythmias to take a DOAC like apixaban or rivaroxaban and to do it until either the risk is deemed to be miniscule due to treatments and interventions or until the disorder remedies itself, apparently, and no further arrhythmias are recorded over time. Either way, this would take months to determine, and it should be on the advice of the person from whom you are receiving expert care.