How to disable multi touch on entire app or just a view using SwiftUI?

No SwiftUI solution but you can use UIKit solution in SwiftUI app.

Use UIView.appearance(). This will disable multi-touch on the whole app.

@main
struct SwiftUIDEMO: App {
    init() {
        UIView.appearance().isMultipleTouchEnabled = false
        UIView.appearance().isExclusiveTouch = true
    }
    
    // Body View
}

Or you can also use UIViewController touch event and maange .isUserInteractionEnabled.

extension UIViewController {
    open override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
        super.touchesMoved(touches, with: event)
        self.view.isUserInteractionEnabled = true
    }
    open override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent?) {
        super.touchesEnded(touches, with: event)
        self.view.isUserInteractionEnabled = true
    }
    open override func touchesCancelled(_ touches: Set<UITouch>, with event: UIEvent?) {
        super.touchesEnded(touches, with: event)
        self.view.isUserInteractionEnabled = true
    }
}