Counting the number of emails sent per day

1. Add a new table (name it whatever you want)
2. Add a linked record field that links to the table containing the Date Email field in your photo
3. Create a record in the new table for each day you send an email and link those records to them
4. Then add a 'Count' field to the new table.

Your other option would be to Group records by date within your current table and use the summary bar.

Group records - https://support.airtable.com/docs/grouping-records-in-airtable
Summary bar - https://support.airtable.com/docs/using-the-summary-bar-in-airtable-views