What are the minimum devices require to build small network ?

Hello

I am not really going to build network (because of cost) . so I am just trying to learn about networking I did google search. I found some useful information to build small network. I found PCs , router, switches, Ethernet cable, modem are common devices that require to make small network. I have query, Suppose I have two PCs, so how many routers and switches are require to make small network?

actually with the right settings you can network 2 computers together without any "middle men".

you can also do it with 3 computers or more, but then one computer needs to act as a "middle man" and have enough ethernet ports available, in a "star configuration"
https://www.jegsworks.com/lessons/computerbasics/lesson7/network-star.png

also similar is true in a loop configuration:
https://www.rfwireless-world.com/images/ring-network.jpg

there are pros and cons to each method, but now a days routers are cheap and really the only thing needed instead of a full computer to do all the work.

also modems can only network from one PC to ISP, which is why the above mentioned is required... although some modems now have routers built in them

Two pc's you can use a crossover cable on the Ethernet ports

In this diagram there is Ethernet cable rj45. this cable allow to share files between two PCs . I think this type of network is wired network

Look at another diagram

I don't understand second diagram. why there is switch, router and modem ?

In the second its a old pic, the router is more of a firewall. It is also there to split the access and control the sharing of the internet.

Modern routers do the modem and sharing bit all in one, the switch is there so all 3 computers can access each other. Slightly more to it than above, but it gives you an idea. Also note the top pic isnt connected to the internet, if you simply want to connect 3 computers then you just need the switch.

"Sneaker Net"
Two computers, one thumb drive, and running shoes.
Put the thumb drive in computer A. Transfer files. Pull the drive. Run to computer B. Incest the drive. Transfer files.
https://en.wikipedia.org/wiki/Sneakernet