Good afternoon
Please i want to know how can i get a consumed data on on interface an also know how to limit internet data on one computer (for example allow the maximuin of 5 Go to the computer )

Thanks

in Interfaces you see all your interfaces. On the right side you see a little black triangle, pointing down (just below the FIND window).

Open that, and select Show columns. Select Tx Bytes an Rx Bytes. The they will show for each interface the amount of traffic. I do not know though, how to reset them or for what period this is.

i have 75 Gb data per month how can i do to know the cosumsion of all computers

while you can check data consumption per interface but there is nothing automatic to remember its data usage after a reboot unless you use a script for this.
I'd suggest to use a hotspot. so each device on your network has to log in to the hotspot to access the internet. they can log in by MAC or you can set the idle time to xx days (so to avoid frequent log in)

Alternatively, if your router is highly unlikely to have a reboot. set up static DHCP for all devices, so that each device will have its own fixed IP, then create simple queue for each IP, then the data usage is shown per simple queue