Edit in JSFiddle

function myViewModel() {
    this.allowedPageSizes = [5, 10, 20, 50, 500];
    this.allowedRegions = [
        { name:"World", code:"WLD" }, 
        { name:"Europe / Asia", code:"ECS" }, 
        { name:"North America", code:"NAC" }
    ];
    
    this.chosenRegion = ko.observable("WLD");
    this.numberToShow = ko.observable(5);
    this.fetchedData = ko.observableArray([]);
    this.queriesDone = ko.observable(0);
    
    this.reset = function() {
        // We're changing two underlying observables, but because of throttling,
        // it will only trigger one Ajax call (the update is effectively atomic)
        this.chosenRegion("WLD");
        this.numberToShow(5);
    }
}

var vm = new myViewModel();
ko.applyBindings(vm);

// Whenever "chosenRegion" or "numberToShow" changes, repopulate "fetchedData"
ko.dependentObservable(function() {        
    $.ajax("http://api.worldbank.org/country?prefix=?", {
        dataType: "jsonp",
        data: { per_page: vm.numberToShow, region: vm.chosenRegion, format: "jsonp" },
        success: function(data) { 
            vm.queriesDone(vm.queriesDone() + 1); // Increment "queriesDone"
            vm.fetchedData(data[1]);
        }
    });
}).extend({ throttle: 1 });

Region: 
<select data-bind="options: allowedRegions, optionsText: 'name', 
                   optionsValue: 'code', value: chosenRegion"></select>
Show: 
<select data-bind="options: allowedPageSizes, value: numberToShow"></select> 
record(s)

<button data-bind="click: reset">Reset</button>

<hr/>
<table>
    <thead>
        <tr><th>Name</th><th>Capital</th><th>Code</th></tr>
    </thead>
    <tbody data-bind="foreach: fetchedData">
        <tr>
            <td data-bind="text: name"></td>
            <td data-bind="text: capitalCity"></td>
            <td data-bind="text: iso2Code"></td>
        </tr>
    </tbody>
</table>

<hr/>
Done <strong data-bind="text: queriesDone"></strong> Ajax request(s)