How many records can be added to an Email Workflow at one time?

There is no limit to the number of total records you can add to a workflow over time. There is however, a limit to the number of records that can be added at one time to the workflow when initially adding records upon workflow setup. 

The limit for adding records at one time is 20,000 records.

The 20,000 record limit is referring to adding Partner or Prospect records to the workflow upon setup. If additional records are added later, they will still be able to enter the workflow as the workflow makes its way through the queue.

For example: When creating a new workflow and adding records, you will be able to add up to 20,000 records at that time. Once the workflow starts running, more records will be able to be added to the workflow.

If you are using a saved view and are continually adding records to that saved view from time-to-time, all the records of that saved view will eventually go through the workflow even if the saved view exceeds 20,000 records.