Is there a way to find out how many of each iPhone model is currently in use?
There is no publicly available data for this information. Apple doesn't (and is not obliged to) make this data public.

If you are an iOS app developer, you can integrate with one of the many analytics solutions available that can give you a limited idea about similar metrics, (but for users of your app).

You can also consider looking at a similar trend report from Mixpanel, one of the leading players in the business. This should help you get an idea about similar if not exact data that you are looking for.

The kind of data you are looking for is not something that's generally made available publicly, even if Apple, or some 3rd party company has access to it.

P.S.: No affiliation whatsoever with Mixpanel.