It will either write only the element or the operation will fail. Ignition writes to OPC array elements using the index range parameter of the Write service to target individual array elements. Some servers don’t support this and it fails. Some do and the write will succeed.
If the server doesn’t support this parameter the only way to write is doing it yourself in scripting, where you have no choice but to write the entire modified array rather than an individual element.
Tag change scripts deliver one event containing the entire array value each time the array value changes. If multiple elements changed at the same time the server would deliver that as one update to the array value, not multiple updates, and you’d see the same one update in your tag change script.