This is not very satisfying, but at least it is a measure of activity of one of its largest sub-groups:
At the height of its influence in 2011 and 2012, Tea Party Patriots was bringing in $20 million a year in contributions and employed 30 people, its tax records show. In 2017 it collected $4.8 million and had a staff of 15.
And at least some Tea Party exponents who thought the Tea Party stood for reducing the deficit, (e.g. Matt Kibbe) have declared the Tea Party dead last year. Some people, on the left in particular, disagree that that was what the Tea Party was mainly about. However, it was still a formally valid characterization: Tea Party Republicans distinguished themselves from the rest of Republicans on the intensity with which they viewed a number of issues. On a 2014 Pew poll, which might not have covered all the possible issues though, the budget deficit was the biggest difference (15 percentage points), followed by ACA (11 points).