Hey, I'm the newbie of Cisco Packet Tracer. In the image as you can see there are lots of computers(0-99,1-200). I couldn't do that. Is it possible or it just some way of describing how much computers are there?
In packet tracer there's really only the computer you can actually see.
It's a principle schema to describe the topology and the core components of the network. The actual number of clients computers is not relevant to understand the network architecture.
As you correctly guessed it's only a way of displaying how many computers are connected in this VLAN but you can't connect 100 computers on a single switch in packet tracer. (So you may assume switch0 actually represents VLAN1 and is distributed on several physical switches)
The ellipse is a standard feature on the latest versions and the "..." is added using the "place note" feature (see screenshot below, keyboard shortcut "N"). It is recognizable due the very light grey background around the dots.
You can drag and drop as many computers you need/want from the "end devices" or something like that. Once you drag them in, you can connect them as you would like. You can get to CLI and that all of the other features. There are some things you can do, but I would encourage you to try it out and see what you can do! There are plenty of YouTube videos that can help.