I am planning to Convert the synchronized data extension (Contacts) to Sendable Data extension. Now when I populate the data extension with contact data will this be counted as a contact and will I be charged? If so, what is the possible work around
There are 2 things here:
Populating data into a sendable data extension doesn't create any new billable contacts, but use the sendable data extension for sending does create if the record contact key isn't existing in All Subscribers List yet.
The data stream between Salesforce CRM and Marketing Cloud will create billable contacts in MC, so when we set up the synchronized contact data extension, it will create contacts in MC if the SF contact record ID isn't existing in All Subscribers List yet.
So if you make a copy DE from the synchronized contact DE for sending, it won't create duplicate contacts since they are already created from the data stream.