How does NAT handles bandwidth?

For example in a large office

there are 80 computers interconnected and uses a single public IP to access internet using NAT..

when those 80 computers tried to browse, does the 80 computers share the whole DSL bandwidth? or are there some protocols pretty much like CIR in which a particular amount of bandwidth can be used by each hosts?

I guess the best answer would be they all share the DSL. If only one computer is trying to access the internet, then they would get all the bandwidth, but if two tried to access the router would forward packets on a first come first serve basis (assuming default configuration)

Process of the router is as follows (at least on a cisco)

1) Client sends packet for the internet
2) Router receives packet and looks it up in routing table
3) Router checks for ACL matches (This is really a two stop process, first check "inside to outside" and then check outbound ACLs)
4) Packet matches NAT ACL so the source IP address is changed and a entry is made in the Nat translation table (can be seen with show ip nat translations)
5) Packet is forwarded.

So to answer your question, it really is more like everyone shares and its kind of First in First out (fifo). If you wanted to prioritize a type of traffic you can do all sorts of fun things with QoS, such as dedicating a certian amount of traffic to a client or service.