Hi davewalter,
You can do this but you would have to create a calculated field, using an expression like:
Iif(Activity.IsChangeOrder == 1,1,0)
This will return 1 if IsChangeOrder = Yes, and 0 if not. You can then use a Summary function to total up the 1's and 0's, resulting in a +1 for every Change Order.
Hope this helps.
- Justin