Hi Carrie, this would be a very difficult feat to track all the images that have been uploaded to Wikimedia Commons and then used in a Wikipedia article.
While there are presently 161,061 images in Wikimedia Commons that have the word “iNaturalist” somewhere in their file metadata, only the first 9k appear to be images from observations, as later search results switch to category listings (not images) and additional results greater than 10k cannot be returned. However, given the priority with which the search engine displays results (word in image file name, word in image metadata, categories, then scanned books) and the requirement to list the source of an image, it seems likely that there are about 9.3k images uploaded from iNat. There is also a module within Wikimedia Commons named iNaturalist2Commons that appears to have been used to automate the uploading of over 3k of these images.
At least 579 images sourced from iNat have been manually verified and a new license-checking bot for images tagged as being sourced from iNat has processed a further 4,852 images since starting operation in July this year. Unfortunately, these numbers do not accurately reflect the actual rate at which new iNat images are being uploaded to Wikimedia Commons as both categories were working off of a backlog of images waiting for review. Additionally, it is unknown what proportion of iNat images uploaded were tagged for this category of image review .
Now for the harder part, once an image has been uploaded to Wikimedia Commons, it may then be featured on multiple Wikipedia projects or none at all. There are over 300 different language Wikipedias that can draw from images stored on Wikimedia Commons. Some language Wikipedias will populate the infobox (or taxobox) on an article automatically with an associated image based on the image links stored in the corresponding Wikidata (another sister project) entry while many others are manually added to each individual article page. The metadata of each image file on Wikimedia Commons contains a list of the different projects that the image is currently being used on (example here). A single Wikipedia, like English Wikipedia, may use the same image in multiple taxon levels. Therefore, the total number of articles on Wikipedia featuring an image originally sourced from iNaturalist could be orders of magnitude lower (or higher) than the number of images estimated to be uploaded to Wikimedia Commons.
Metrics are useful to track the progress of an activity, but can be difficult to measure when not designed into the system from the start. In this case the process is not straightforward, or even a single process that is identical between different users. While we may be able to determine a lower bound of the total number of images uploaded, the number currently being used in articles may not be estimated so easily.
If you would like, I can contact the authors of the iNaturalist image review bot on Wikimedia Commons to see if they have any additional insight into the total numbers and rate of iNat images uploaded to Wikimedia Commons. However, to determine the numbers of images used within Wikipedia articles you would have to write a script that reviewed the metadata of each image file on Wikimedia Commons that was tagged as being sourced from iNat and then counted the number of articles listed.