I have two question about the networks.. just let me know if i am right..

lets say I have a requirement (worst case scenario) of supporting up to 5000 concurrence connections

lets say all request are for simple html page with size of 300KB.( I know about cpu, and ram etc.. I am just want to understand the network side)

thats mean that I need 300x5000 = 1500000KB = 1500Mb..
what kind of network I need to support this kind of users?

when linode say 125 Mbps does it mean that server can send 125mb per second? so to support 1500Mb for second i need at least Linode 16GB(2000 Mbps) ?

2) does each linode get is own speed, or its add up to all the linode I have?
I have 1 node balancer
2 Linode 1GB - php+nginx
1 linode 2gb - mysql
1 linode 1gb - redis

total of 4 linodes so I am having 625Mbps network out or only the linode that gets the traffic are counting to my network speed?
so only the php linode gonna get the traffic so i have 250Mbps? cant understand it, thanks a lot

Each Linode has it's own network speed outbound cap. They won't get pooled persay. If you put a NodeBalancer in front of them however, you can split up the traffic.

Your math seems to be on the right track but it gets a bit more complicated.
1) Not all 300KB of that page would be downloaded in a single second (most likely) meaning that number would drop a bit.
2) Anything cached by the client would not be downloaded again.
3) Any external resources, say Google Analytics, jQuery, etc would be loaded from a different server thus not counting towards your outbound speed.

thanks I know about multiple resources caches and etc.. just gave a simple number to check if I am getting it right..
lets say I have 1 html page, with no dynamic patrs only html and text(no images) that is size is 300kb

because all of my resources are on external cdn, and nginx cache all my php 99.9999% time, my limitation factor is the network(I tested it). and I am on limited budget

so If network speed not get pooled, and I need big network speed than I rather have 1 8GB linode with 1000Mbps Network Out than split each service to his own linode.
but than I am losing my scalability

1000 Mbps = 1gb out of data every second or its 100mb out data per second?