store arraylist in textbox one bye one using loop

This has nothing to do with SQL Server. Also, we don't need to reply 'ASAP', we are happy to help, but we are certainly not required to do your work for you.

You should use List<string> and not ArrayList, which has been obsolete for over a decade.

This[^] is the method you use to take an array and turn it into a single string. But, even

StringBuilder sb = new StringBuilder();

foreach(string s in myArrayList)
(
  sb.Append(s);
  sb.Append(", ");
)

string result = sb.ToString();

will work. If you don't know how to do that, you should not be using ASP.NET, you should be working through a basic book on C#.