grammatical number - Can I use "an" before plural noun?

I wrote in my manuscript that;
"Additional 17 human samples not preserved in formaldehyde were provided for age dating."

But my English proofreader corrected that
"An additional 17 human samples not preserved in formaldehyde were provided for age dating."

She explained the reason as "I added an 'An' here because you are starting a sentence with this information (and the article here can help in emphasizing the action that is going on in this sentence). "

I need further explanation.