You could match that number using the following code:
string search = "<iframe width=\"100%\" height=\"450\" scrolling=\"no\" frameborder=\"no\" src=\"https://w.soundcloud.com/player/?url=https%3A//api.soundcloud.com/playlists/26104012&auto_play=false&hide_related=false&visual=true%22\"></iframe>";
string sPattern = "^.*src=.*playlists\\/([0-9]+)&.*$";
Match match = Regex.Match(search, sPattern, RegexOptions.IgnoreCase);
// Here we check the Match instance.
if (match.Success)
{
// Finally, we get the Group value and display it.
string id = match.Groups[1].Value;
}