Verified with Sat 6.10.0 snap 2.0. Imported a manifest, created a custom product, added all of those to the activation key, registered a host using that activation key. Then WebUI -> Report Templates -> "Subscription - Entitlement report" -> Generate -> Submit. The resulting file contains column "Subscriptions consumed" (next to "Subscription Quantity") which contains a number of those subscriptions consumed by all hosts together. The report takes into account all the subscriptions, including custom ones, which can be told apart by having Contract column empty.
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory (Moderate: Satellite 6.10 Release), and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. https://access.redhat.com/errata/RHSA-2021:4702