if I used 1769-L33 PLC as main station , the small HMI's will read different tags in PLC (each HMI read 6 bit memory tags for valves state , write 4 integer words) its not big data
Only the Main HMI have larg number of tags and I will connected it via rs232 port with main PLC 1769-L33
Its not really very important if the small HMI´s are reading a small number of tags or a large number of tags it will use a similar amount of Network Bandwidth.
Second the 1769-L33 does not have a Serial port built in. In order to use a serial panel view (which I wouldn´t recommend) you would need to add a 1769-ASCII card. It would be better for you to use ethernet and not add the ASCII card.
I would investigate the 1769-L16ER-BB1B. From memory the difference in price between it and a 1734-AENTR is quite small, it might be worth looking into.