使用 UIApplication.shared.applicationIconBadgeNumber 可以设置应用程序右上角的提醒个数。

import UIKit
import UserNotifications

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    
    var window: UIWindow?
    
    func application(_ application: UIApplication, didFinishLaunchingWithOptions
        launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        //请求通知权限
        UNUserNotificationCenter.current()
            .requestAuthorization(options: [.alert, .sound, .badge]) {
                (accepted, error) in
                if !accepted {
                    print("用户不允许消息通知。")
                }
        }
        
        return true
    }
    
    func applicationWillResignActive(_ application: UIApplication) {
    }
    
    func applicationDidEnterBackground(_ application: UIApplication) {
    }
    
    func applicationWillEnterForeground(_ application: UIApplication) {
    }
    
    func applicationDidBecomeActive(_ application: UIApplication) {
    }
    
    func applicationWillTerminate(_ application: UIApplication) {
    }
}

--- ViewController.swift ---

import UIKit

class ViewController: UIViewController {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        //设置应用程序右上角的提醒个数
        UIApplication.shared.applicationIconBadgeNumber = 78
    }
    
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }
}