JSFiddle - React, Tailwind, and code Playground

HTML

<form action="<?php echo $editFormAction; ?>" method="POST" name="contract" id="form1">
    <table class="myp-table">
        <tr>
            <td>Value Test</td>
        </tr>
        <tr>
            <td>
                <select name="playerContract[]">
                    <option value="0">0</option>
                    <option value="1">1</option>
                    <option value="2">2</option>
                    <option value="3">3</option>
                </select>
            </td>
        </tr>
        <tr>
            <td>
                <select name="playerContract[]">
                    <option value="0">0</option>
                    <option value="1">1</option>
                    <option value="2">2</option>
                    <option value="3">3</option>
                </select>
            </td>
        </tr>
        <tr>
            <td>
                <input id="submit" class="linkbuttonmp" name="contract" type="button" value="Invio" />
            </td>
        </tr>
    </table>
</form>

JavaScript

var max = 5;

$("#form1 select").change(function () {
    
    var selects = 0;

    $("#form1 select").each(function () {

        selects = selects + parseInt($(this).val());

    });

    
    if(selects >= max) { 
        $("#submit").attr("disabled","disabled");
    } else {
        $("#submit").removeAttr("disabled");
    }

});