string str ="abcdefg1234567qwertyuzxcvbnm";

Console.WriteLine(str);

void Main()
{
    string str ="abcdefg1234567qwertyuzxcvbnm";
    string result = SplitStr(str, 7);
    //result.Dump();
}

// Define other methods and classes here

public string SplitStr(string str, int maxSymbols)
{
    var sb = new StringBuilder();
    var counter = 0;
    foreach (var element in str)
    {
        if (counter == maxSymbols)
        {
            sb.Append(" ");
            counter = 0;
        }

        sb.Append(element);
        counter++;
    }
    return sb.ToString();
}

string str = "abcdefg1234567qwertyuzxcvbnm";
string res = Regex.Replace(str, ".{7}", "$0 ");
Console.WriteLine(res);