Using any Smart Campaign Salesforce flow step consumes one API call per record, so using the Add Member to SFDC Campaign flow step will use as many API calls as records you are adding to the SFDC Campaign flow. To reduce the number of API calls used and ensure members are added to SFDC Campaigns, create a Marketo Program (if one doesn't already exist) and sync it to the existing SFDC campaign via the Salesforce Campaign Sync - more information here. This functionality utilizes the back-end Salesforce sync (which is much more efficient) allowing ~200 records to be synced to SFDC and added to the associated SFDC Campaign.
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