I knew I had seen. Feature in Swift strings that could automatically pluralize words depending on a value. Took me a while to find the actual thing I wanted. Turns out it's called morphology. Jordan Morgan has a great post about it.
It works nicely when using Text
in SwiftUI:
Text("Last compared \(timestamp.formatted()) and seen ^[\(historyItems.count) time](inflect: true)")
It's not the nicest syntax, but better than doing it all myself.