Minefield, random bombs

Good afternoon I'm having a problem using the random method to try to show only 10 bombs in my mine field, except that it appears without limits. Does anyone know where I'm going wrong?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Camp
{
    class Program
    {

        static int[,] matriz = new int[10, 10];

        static void Main(string[] args)
        {
            PreencherMatriz();

        }

        static void Bomba()
        {
            Random rand = new Random();
            int linhaRand = 0;
            int colunaRand = 0;
            //bool minValue = false;
            bool maxValue = false;
            int bomba = 0;
            do
            {
            bomba ++;
            linhaRand = rand.Next(1, matriz.GetLength(0) - 1);
            colunaRand  = rand.Next(1, matriz.GetLength(1) - 1);
            if ( matriz[linhaRand, colunaRand] == '9')
            {
                maxValue = true;
            }
            }while(!maxValue && bomba <= 1);


            matriz[linhaRand, colunaRand] =0;
            LerMatriz();

            Bomba();


        }

        static void LerMatriz()
        {
            Console.Clear();
            for (int i = 0; i < matriz.GetLength(0); i++)
            {
                for (int j = 0; j < matriz.GetLength(1); j++)
                {
                    Console.Write("{0} ", matriz[i, j]);
                }
                Console.WriteLine("");
            }
        }

        static void PreencherMatriz()
        {
            for (int i = 0; i < matriz.GetLength(0); i++)
            {
                for (int j = 0; j < matriz.GetLength(1); j++)
                {
                    if(j == 0 || j == matriz.GetLength(1) - 1)
                    {
                        matriz[i, j] = 5;
                    }
                    else if(i == 0 || i == matriz.GetLength(0) - 1)
                    {
                        matriz[i, j] = 5;
                    }
                    else
                    {
                        matriz[i, j] = 9; 
                    }
                    Console.Write("{0} ", matriz[i, j]);
                }
                Console.WriteLine("");
            }
            Console.ReadKey();
            Bomba();
        }
    }
}