ok maybe iPad and apple watch doesn't count but still multiple iPhones belonging to the same user counts as separate install. And it's not only unique first install:
"A first annual install may result from an app’s first-time install, a reinstall, or an update from any iOS app distribution option — including the App Store," [0]
so updates and reinstalls count as well - just all installs, reinstalls, updates in 12 month period counts as one. But next year if you just make a bug fix to still support new iOS version that update will be paid as well to all your user install base.
"A first annual install may result from an app’s first-time install, a reinstall, or an update from any iOS app distribution option — including the App Store," [0]
so updates and reinstalls count as well - just all installs, reinstalls, updates in 12 month period counts as one. But next year if you just make a bug fix to still support new iOS version that update will be paid as well to all your user install base.
[0] https://developer.apple.com/support/core-technology-fee/