The best practice is to use a single Chargent Order for recurring billing. This is important because the system references when the last recurring transaction related to that record was, in order to see when the next charge should be made.
Whether you use multiple records for additional one-time charges is more of a business process decision. Many Chargent customers run one-time charges from the same record simply by modifying the "Charge Amount", clicking Save, and then Charge. Manual charges are separate from recurring payment schedules, so it should not affect existing recurring payments, if any are set up.
Some organizations prefer to have a new Chargent Order record for each different purchase. In that case, you would need to populate the credit card data in the new record via one of three methods:
1) Use an Apex trigger to copy the data
2) Send the customer a Payment Request for them to enter the data
3) Turn on "View Encrypted Data" permissions in a Profile and then "Clone" the original record
Number 3 is not recommended for security purposes. However if you clone a Chargent Order any encrypted fields will not be copied over otherwise.