Write the count files in folder to a spreadsheet

I'm doing a mail merge from a Google sheet, and outputting the PDFs to a Google folder. I'd like to be able to count the files, to ensure that they've all been merged.

I hacked a Google Apps Script:

var sheet = SpreadsheetApp.getActiveSheet();
function countfiles () {
   var folder = DriveApp.getFolderById("1-8Ek9gkm-_-YYYLK5_x7Y3Do_CzvmMfF");  
   var contents = folder.getFiles();
   var i=0;
   var contentsArray = [];
   while (contents.hasNext() && i < 5000) {
     var file = contents.next();
     contentsArray.push(file.getId());  //this is the array that will have the Id's of the first 4999 files.
     i++
   }
      sheet.clear();  
      // Insert Header
      sheet.appendRow("contents")
   }

I'm not sure how to output to my sheet the total count.

Okay, take a look at the Sheet's getRange() and Range's setValue() methods. First, for getRange() we define a range in A1 notation (you can also define it other ways) to get a single cell as a Range object. Then we use the setValue() method on that cell to set the value!

var sheet = SpreadsheetApp.getActiveSheet();
function countfiles () {
  var folder = DriveApp.getFolderById("1-8Ek9gkm-_-YYYLK5_x7Y3Do_CzvmMfF");  
  var contents = folder.getFiles();

  var i=0;
  var contentsArray = [];

  while (contents.hasNext() && i < 5000) {
    var file = contents.next();
    contentsArray.push(file.getId());  //this is the array that will have the Id's of the first 4999 files.
    i++
  }
  sheet.clear();  


  var range = sheet.getRange("A23"); //Get range object of single cell.
  range.setValue("Number: " + i); //set the value as count (i).

  // Insert Header
  sheet.appendRow("contents")
}