Hi,
You need to write code to achieve that e.g.
AEM - Touch UI Component Dialog Field Regex/Custom Validation
You can write this code on event of Add button
$inputEle = $("your multifield item")
var multiFieldItem = $inputEle.find("coral-multifield-item").size();
var multiFieldbutton = $inputEle.find(".coral-Button")
if (multiFieldItem > max) {
// disabled add button using javascript
multiFieldbutton.attr("disabled", "disabled");
}
Arun Patidar