This code will help you..................

function dynamicoptionset() {

//get value in no in variable
    var totaldivision = Xrm.Page.getAttribute("fieldname" ).getValue();
    if (totaldivision == null || totaldivision == "" )
        return;
    else {
        var optionsetControl = Xrm.Page.ui.controls.get( "new_division" );
        var options = optionsetControl.getAttribute().getOptions();

        optionsetControl.clearOptions();
        for ( var i = 0; i < totaldivision; i++) {

            optionsetControl.addOption(options[i]);
        }
    }
}


SOURCE : JUST2CODE.IN
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