How on Roblox do you change the number of people in a server

I assume you mean change the max amount of players that can join a server? Well this is simple to do.

1. Open your place in ROBLOX Studio
2. Go to the Explorer panel (Where all the objects in the game can be found).
3. Find the service named Players and select it.
4. Go to the Properties panel and find the value MaxPlayers
5. Change the value there from 12 (This is the default value) to how ever many players you want to be in the server at one time.
6. Save your place and publish it to ROBLOX.

Now your done!
Note that if you go into 'Build mode' then you will have to click the Tools button at the top of the screen to find the Explorer panel and etc.
Also if you can't find the panels even after that then go to View and select Explorer and Properties.