Umrah mufradda can only be performed once? We cannot perform it more than once?

Yes, that’s why it’s called Umrah mufradah, meaning one single Umran. That law of course apply if you are performing umrah in one month. But if you are let’s say performing Umrah at the end of January then when February started you wanted to perform another umrah you can. You can also do another umrah if it’s in behalf of someone else after having done your umrah.