JSFiddle - React, Tailwind, and code Playground

JavaScript

var jsonObj = [
{
    "MNGR_NAME": "Mark",
    "MGR_ID": "M44",
    "EMP_ID": "1849"
},
{
    "MNGR_NAME": "Steve",
    "PROJ_ID": "88421",
    "PROJ_NAME": "ABC",
    "PROJ_ALLOC_NO": "49"
},
    {
    "PROJ_ID": "88421",
    "PROJ_NAME": "ABC",
    "PROJ_ALLOC_NO": "49"
    }
];

var count = 0;
var i;
for(i = 0; i < jsonObj.length; i += 1) {
    if(jsonObj[i]["MNGR_NAME"]) {
        count++
    }
}

document.writeln(count);