Theoretically a gigabit network will allow 1,000 Mbps - see
wikipedia for details
NB this link takes you directly to the details on 1000BASE-T which applies to gigabit over copper cable specs and is probably therefore the most relevant.
However, in the real world, you are unlikely to achieve this (managing 20 megapixel camera setup you describe) with any stability with an
ordinary PC.
An ordinary PC with gigabit networking will probably be fine for most user's system growth up to about eight cameras (there are, as you saw from the calculator, many variables).
IMHO the first variable to adjust, in order to allow your 'ordinary PC' to cope, is the video frame rate. For many years with analogue CCTV, multiplexers, and VCRs the best frame rate we could deliver
for all the connected cameras was 6 frames per second e.g. total for the system, not per camera.
Of course, the term 'ordinary PC' is not very scientific in itself, and it is possible to put together PCs with fantastic specs now.
Dedicated NVRs (
Network Video Recorders) would have at least two gigabit ports (that's two ports), which allows some degree of sharing of network bandwidth (effectively you can connect to two networks at the same time, both being load balanced to not have too many cameras connected). Bear in mind that this CCTV workstation is unlikely to be an isolated box i.e. you'll probably also want to allow bandwidth capacity for other users (clients) to connect to it and access its live and recorded images.
Of course, it's not just a matter of getting all that data across the network and into the box, it then needs to be processed, displayed, stored ... So the box needs to be up to that job too e.g. bus speeds, memory speeds, disk access & write speeds ...
Take a look at the
datasheet specs for these souped-up rack-mount IPCCTV Recording PCs to get an idea of the components and specifications required.
As a guide, those boxes will run from about £1,600 to well over £5,000.
There are many ways of skinning this cat, key to achieving a solution which meets your requirement well is our old friend
'The Operational Requirement' - what exactly are you trying to achieve!? You need to know your end goal before starting out.
Most people contemplating a self-design/install do not have
twenty megapixel cameras on their expected growth plan.
Therefore, they start off with a good spec PC, a gigabit network, suitable CCTV software, a terabyte of HDD storage in the box, and a few IP CCTV cameras ... all according to budgets and scale of issues being addressed ...
If you also bear in mind the basic cat5 cable length limitation of 100m, it's quite common to have zones of multiple cameras recording to one
local PC replicated multiple times across a large site.
Hope that helps a little, feel free to pursue any details ...