Turns Out It Was 12 Reps...

So I had the session programmed in as 10 x 400m with 200m recovery. The pace I'd set was about right. Over the first four reps my heart rate crept up and stabilised out at 167 at the end of each one. I was happy to see my way to the tenth. Only then did I realise there were two more to go. My heart didn't sink as I felt I could manage them. All in all a good session.