At  serial port example page there is example code. Look:

int numRead = 0, numReadTotal = 0;
char buffer[50];

for (;;) {
    numRead  = serial.read(buffer, 50);

    // Do whatever with the array

    numReadTotal += numRead;
    if (numRead == 0 && !serial.waitForReadyRead())
        break;
}

But serial.read() function returns qint64.

I suggest make
qint64 numRead = 0, numReadTotal = 0;

P.S. I agree that in real app serial.read() never returns more than int. If you think so you should use numRead=(int)serial.read(buffer, 50)