AlchemyLanguage used to return the number of API transactions that took place during any call, this was particularly useful when making a combined call.
I do not see the equivalent way to get those results per REST call.
Is there any way to track or calculate this? I am concerned about things like some of the sub-requests like when you ask for sentiment on entities does that count as two, or one plus an additional call per recognized entity?
There's currently no way to track the transactions from the API itself. To track this (particularly for cost estimates), you'll have to go to the usage dashboard in Bluemix. To find it: sign in to Bluemix, click Manage, then select Billing and Usage, and finally select Usage. At the bottom of the page you'll se a list of all your credentialed services. Expanding any of those will show the usage plus total charges for the month.
As far as how the NLU service is billed, it's not necessarily on a per API request as you mentioned. The serivce is billed in "units" and from the pricing page (https://console.ng.bluemix.net/catalog/services/natural-language-understanding):
A NLU item is based on the number of data units enriched and the number of enrichment features applied. A data unit is 10,000 characters or less. For example: extracting Entities and Sentiment from 15,000 characters of text is (2 Data Units * 2 Enrichment Features) = 4 NLU Items.
So overall, the best way to understand your transaction usage would be to run a few test requests and then check the Bluemix usage dashboard.
I was able to do a simple test, and made calls to a set of highlevel features and included sub-features. And it appeared to only register calls for the highlevel features.