Check number

Use a integer to count the waves, spawn, then reset it.

  using System.Collections;
  using System.Collections.Generic;
  using UnityEngine;
  
  public class WaveSpawner : MonoBehaviour
  {
      public GameObject[] enemys;
      public GameObject[] bosEnemy;
  
      public float timeBetweenWaves;
  
      public int WaveCount; //could use this, but not sure if you want to spawn only a boss/boss + wave
      public int WaveCountBoss;
      public int enemyCount;
      public int spawnedEnemys;
      int randomInt;
  
      void Update()
      {
          if(enemyCount == 0)
          {
              Invoke("SpawnRandom", timeBetweenWaves);
              WaveCount++;
              WaveCountBoss++;
          }
      }
  
      void SpawnRandom()
      {
          for (int i = 0; i < spawnedEnemys; i++)
          {
              randomInt = Random.Range(0, enemys.Length);
              Instantiate(enemys[randomInt], transform.position, Quaternion.identity);
              enemyCount++;
          }
          if (WaveCountBoss >= 10)
          {
             WaveCountBoss = 0;
             //spawn boss here
          }
          //every 10th wave it should spawn an bosEnemy
      }
  }