You can convert e to a Number and add one to it like in this fiddle.

function html2json() {
  var json = '{';
  var otArr = [];
  // var i = 1;
  var tbl2 = $('table tbody tr').each(function(e) {
    x = $(this).children();
    var itArr = [];
    x.each(function() {
      itArr.push('"' + $(this).text() + '"');
    });
    otArr.push('"' + (Number(e) + 1) + '": {' + itArr.join(',') + '}');
  })
  json += otArr.join(",") + '}'

  return json;
}

The json you're returning is not valid though. You may want to do something like this fiddle if you can to simplify and ensure valid json and to create your objects from any table structure.

function html2json() {   
  var otArr = [];
  var tblHeaders = Array.from($('table thead tr')
    .children())
    .map(header => $(header).text());  
  var tbl2 = $('table tbody tr').each(function(e) {        
    const values = Array.from($(this).children());
    const row = {};
    for (let i = 0; i < tblHeaders.length; i++){        
        row[tblHeaders[i]] = $(values[i]).text();
    }
    otArr.push({
        [e+1]: row
    })      
  })
  json = JSON.stringify(otArr);  
  return json;
}

Synthesizing other answers:

1. Your wire format doesn't have to be the same as your in-memory format.
2. Profile which is better - see if it makes a difference.
3. Simpler is usually better to start with.

Further:

• If you just have a page of results, and few users, then the 2nd format may be no worse than the 1st format.
• If your data is quite sparse, the 2nd format may well be better.
• If you're sending 1000's or rows of data, and you have millions of users, then it's possible that the size of data you send can start to matter, and perhaps the 1st format may help.
• You can't guarantee that all user agents support gzip / deflate, so bear this in mind.