Yes @Andrei G. is right .. you need to use @state variable as structs are value types ..

struct SwiftUIDemo: View {
    @State var tapCount = 0

    var body: some View {
        Button("Tap Count: \(tapCount)") {
            self.incrementCount()
        
        }
    }

    func incrementCount() {
        self.tapCount += 1
        print(tapCount)
    }
}