Simple Python script to sort and make json data unique
I'd recommend making use of a set, which stores only unique elements. That way you can just count insertions to get the total (with duplicates) and the unique ones are easy to retrieve.

Instead of iterating through unique emails and printing them with a comma and a space as a terminator (which gives you the odd comma and space at the end of the emails), make use of ", ".join().

Lastly, eMails should be emails in terms of punctuation, constants should be all caps, and camelCase is discouraged. For clarity, I changed the variable names to be more expressive.

Here is my complete solution:

import os, json

ORDERS_DIRECTORY = "C:\dev\orderArchive" # Reformatted constant.
unique_emails = set() # Holds only unique emails by design.
total_emails = 0 # Boring running total

# json objects stored in multiple files, itterate over all of them
for filename in os.listdir(ORDERS_DIRECTORY):
    with open(os.path.join(ORDERS_DIRECTORY, filename)) as data_file:
        data = json.load(data_file)
        unique_emails.add(data["details"]["email"]) # Make use of set here.
        total_emails += 1 # Corrected spacing here.

print("Comma dilimited e-mails: \n")
print(", ".join(unique_emails)) # Make use of join here.
print("\n\nNumber of unique e-mails is: {} (originally {})".format(len(unique_emails), total_emails))


On a side note, you may want to consider having some more error handling for the email field not being there, file not containing json etc. I'd also consider making the directory a program argument.

Update: fixed ORDERS_DIRECTORY consistency, use of os.path.join(), and str.format() per comments.