When RXF# is low there is at least 1 byte available to be read. No indication is available of how many bytes are available.
RXF# and RD handshake with each other to transfer each byte. If a byte is read by setting RD# low then high again, RXF# will always go high. If there is more data to be read then RXF# will go active low again a short time later.
Please refer to the end of the document Debugging FT232BM and FT245BM Based Designs on the Application Notes page of the FTDI website for some example oscilloscope traces of communication with the FT245BM.