As I have no budget to hire 200 singers, I would like to know what techniques and plugins I could use to, from a few recorded voices, create a multi-singing effect as if it were in a stadium?

Use sound on sound recording mode, and record each part and just start stacking parts on parts.  Pan as needed.  If you can't rerecord the vocals then you have fewer realistic options.  You can clone each part and process each part differently and/or nudge each part for timing offsets.  Either way, one person can record a limitless number of tracks.

http://www.quikquak.com/prod_crowdchamber.html
 
There may be some free ones too. go a googlin.

This was recently posted by Waves, maybe something in it will be helpful or inspire you in some way creatively.
https://www.youtube.com/watch?v=8LrTB9JFBME

Lots of multy tracking and vary the distance to mics considerably. Stereo place the mics and leave them static -  literally vary which side you sing from. Vary your voice characteristics, pretend you're from another nationality and sing it in your language. Some tracks be a bit meek  - some tracks scream your head off. I wouldn't suggest you use plugs at all. It's the variation and missed timing that sells it.  My opinion only - but I've done it a few times.

I've used Clone Ensemble for years: http://www.cloneensemble.com/cl_main.htm
 
Unfortunately it's still 32 bit, but it works well with JBridge.
 
Not only is it great on vocals, it's great as a doubler plugin on other instruments. I've used it as a wall of guitars in the past, which works great as long as you don't go mad on the number of voices.

You might want to demo MUnison from Meldaproduction. To me, it sounds kinda fake on voices, but if the synthesized voices were set back in the mix from the real voices, it might work. 

I would duplicate the audio track and insert a mix of different EQ, pan, and delay to each of the duplicated tracks.  I suggest a delay plugin as opposed to shifting the audio in time, as the delay plugin is better at avoid phase issue. I would bus the original plus the "clones" and compress that slightly, maybe 2:1 to glue the individual tracks together.

Copy the few recorded vocals and adjust the formants w/melodyne

In the past, I've tried loads of different methods to try to fake a choir. All of them sounded fake.
 
IMHO, nothing comes close to Clone Ensemble. It may be 32 bit and look like something from the 90's, but listen to what it does with just ONE voice:
 
http://www.cloneensemble.com/audio/cloneens.mp3
 
And if you don't like having 32 bit plugins in your project, just bounce the audio and delete the plugin.
 
For $25, it's a steal.