Angular: Empty Fiddle

http://angularjs.org/

HTML

<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.13/angular.min.js"></script>
<div ng-controller="MyCtrl">
    <form name="myForm">Availability:
        <select ng-model="selectedAvailability" ng-options="r.name for r in availability" ></select>
        <br><br>
                Enter a number:
        <input name="value" ng-model="enteredValue" type="number" min="0" max="{{getMax()}}" />
        <br> <tt>value = {{value}}</tt>

        <br> <tt>value.$valid = {{myForm.value.$valid}}</tt>

        <br> <tt>value.$error = {{myForm.value.$error}}</tt>

    </form>
</div>

JavaScript

var myApp = angular.module('myApp', []);

function MyCtrl($scope) {

    $scope.availability = [{
        name: 'Available'
    }, {
        name: 'Unlimited'
    }];
    
    $scope.selectedAvailability = $scope.availability[0];

    $scope.getMax = function () {
        console.log('getMax');
        if ($scope.selectedAvailability.name === 'Unlimited') {
            console.log('unlimited');
            return 500;
        } else {
            console.log('available');
            return 100;
        }
    }
}