The innstanceManager has a count property which will give you how many occurances of that subform that exists. Also if you know which occurance that you want to look at you can address the field by using the full somExpression and giving the occurance number on the subform. Something like this:
The issue is that myCount is not getting resolved beacuse it is inside of the quotes and is being interpretted as a string. I believe the expression should be: