one

Not one the answer is more than one

You can add as many web.config file to one solution.

In single application v can have more than 1 web config
files.But only one web config file can be placed in root
folder.We can place web cofig files in sub folders

by default there is only one web.config file in asp.net 
application. but if we want to create more than one 
web.config we can create, but they should not be in same 
directory.

We can add only one Web.config file for each folder in the 
solution. i.e. If we has n folders we can add n + 1(this is 
for solution) Web.config files

By default there is only one web.config file in asp.net 
application. but if we want to create more than one 
web.config we can create, but they should not be in same 
directory.

more than one but only one machine.config

we can create more than one webconfig file for application
but those should not be in same folder i.e only one
webconfig in file in each folder.......