<<< what about Xp Pro-how many could supported with the normal Router/Modern we use for home networking? >>>
That would probably be restricted by the router, rather than the operating system. If the router doesn't specifically restrict the number of clients it can serve, a subnet mask of 255.255.255.0 will restrict the network to 253 computers.
192.168.0.0 is the address for the entire subnet 192.168.0.1 is the router's address 192.168.0.255 is the 'broadcast' address for all hosts in the subnet.
That leaves 253 addresses for computers.
Using a 10.0.0.0 range with a subnet mask of 255.0.0.0 will support a much larger network. I'm not sure of the exact range but something like 10.0.0.2 to 10.255.255.254 if you want to work out how many that would be (if you could physically connect them!)
I heard that XP Home is incapable of hosting a network. I know when I tried it, I failed to get it up and running. Yet I have set up networks with hosts on Win2000 and XP Pro with no trouble at all. I'd definitely go for XP Pro!
<<< I heard that XP Home is incapable of hosting a network. >>>
I think you need to be careful about the terminology, in particular "hosting". Windows XP Home can participate in a Peer-to-Peer network of up to 5 computers and can support incoming connections from 5 computers.
XP Pro can support up to 10 simultaneous incoming connections, which doesn't necessarily limit to 10 the size of the network that it belongs to. XP Pro can also participate in Client/Server based networks, as well as Peer-to-Peer networks, and these will generally be much larger than 10-PC networks.