I've heard people saying they've received multiple acknowledgement letters
When you receive it for the 1st time, Unique Client Identifier and Application Number will be stated with the date they've received your file.
Generally acknowledgement letter tells you the status of your application.
And, you get 1st acknowledgement letter once they open your file for reviewing. Hope this helps

I've received only one acknowledgement letter so far. So people who've got more than one can share their view as well