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.

[0] https://developer.apple.com/support/core-technology-fee/