Hi,
 
This is directed especially to EDM...
 
1) Is it worth using a multiband limiter?
2) How much headroom in needed before the limiter?  What should my RMS level be before limiting?
3) How much limiting is allowed?
 
Would like input, especially from my idol, ilan Bluestone (that's if he participates in the forum discussions)

It doesn't really matter whether it is EDM or not.
 
1.  Used to be a multi-band limiter wasn't thought necessary.  More than a few people use them today.  What do your ears tell you?  I use a single band comp limiter but others here and elsewhere have made good arguments for multi-band.
 
2.  If you use a limiter, you need a few dB (at least) of headroom.  Otherwise, why bother if the song is already at 0 dB?  Usually I just worry about peaks in mixing - -6 dB to -3.  And I use the mastering step to squash those down some so the overall volume average comes up.  I don't push digital mastering hard: only a couple of dBs reduction. 
 
3.  Of course you need to push harder if your mix is quiet.  But again, pushing digital can get harsh (quicker than hardware, to my ears), so it is really a balance between clarity of a mix and overall volume needed.  Your master shouldn't sound obviously weak compared to the next song, and the human ear is sensitive to small changes in vol.
 
If my answers sound flippant, it is because questions like these are like asking "how much pepper do I put on my steak." Different amounts for different folks, as well as cuts of meat.  It is fine to ask others how they do things or their general settings and try those, but it doesn't mean you need to do it exactly like that.  I don't hardly cook with salt, so my wife pours salt on whatever I make.  But I don't.  Who is right? It is mostly a matter of taste. 
 
There are general rules, like you master an mp3 to -.3 dB or the conversion is likely to redline, or if you need to push your limiter more than 6 dB you might want to reconsider the volume of the mix.  But one man's formula can be your mistake.  
 
@

EDM is typically compressed more aggressively than other genres. Only hip-hop is squashed more than EDM. Being neither a practitioner nor fan of either genre, I can only give hypothetical advice from a distance.
 
Contemporary EDM's dynamic range is often less than -8dB, although I understand there is a movement afoot to get back to wider dynamics. Well-respected mastering guru Ian Shepard makes a compelling case for more conservative bus compression for the genre.
 
If I was pressed into service to master an EDM piece, I'd start by inserting the limiter and setting the brickwall limit to -0.3 dB, then bring up the input level until the output hits whatever target RMS value I'd chosen. It's either going to sound good, or not. If it doesn't sound good, then maybe my RMS goal was overly aggressive.
 
What that input level will be is impossible to say, as it depends on the mix. You might need your limiter to give an average compression of only a couple dB if the mix is already loud. As a general rule, you want to give your limiter some room, regardless of genre. If you see the amount of compression being constantly more than, say, 6dB then it's possible your mix is too loud to begin with and you've left the limiter no room to maneuver. For that reason, your unmastered mix's loudness should be conservative, peaking at less than -1dB with an RMS value less than -6dB (-12dB for non-EDM genres).
 
Multi-band compressors (or multi-band limiters) are mostly used to squeeze out the last dB or two. However, a multi-band compressor in front of a broadband limiter can be used with gentle settings and still enhance bass/kick punch. Check out the free Kotelnikov compressor from TDR, not the easiest plugin to learn but once you figure it out it'll be quite effective in this role.
 
Watch the following video closely. You might want to play it more than once, as there are several nuggets of wisdom in there.
 

I do a lot of heavy beat based stuff (what I would describe as electronica with the occasional EDM element) and I have to say I'm always a little disappointed with what a limiter does, even with only a couple of dB of gain reduction. I guess my ears are really sensitive to even a small loss of dynamics, but I know that if I wanted to go commercial with my work I would have to go somewhat louder. Maybe I just to experiment more with release times.

Waves have a useful video on using their L2 limiter.

What they suggest is using the link function to link the threshold and limit/output level sliders, then pull both down until the mix starts to sound wrong. Then back up again till it sounds right. Then set the limit to what you need it to be, typically 0.3dB.

A similar technique can be used with any limiter, even if threshold and limit/gain can't be linked. Just set the limiter on the low side so you don't blow out your monitors or ears then adjust threshold to just before where things go bad.

Personally I rarely need the master bus final limiter to be doing more than boosting overall volume while shaving a very few dB off the peaks. The more tracks there are in the mix, the less the final limiter has to do, because more tracks tends to mean a louder mix anyway.

Multiband compression/limiting can be useful as a frequency balancing tool before a gentle final limiting. In a way they're best though of as eqs that can prevent a particular frequency band from swinging wildy in gain or having so much power in them that they trigger the brick-wall limiter so much that other frequencies are getting wiped out.

As for the gap between RMS levels and peak, it really depends on how you want the finished thing to sound.

Thanks for the great input...
 
Basically, what i want to achieve is this : https://www.youtube.com/watch?v=FFrvXPdjAr8

There's more than limiting going on in that example. In particular, sidechaining a master bus compressor plays a big part. That's what's causes that pulsing effect. That, or possibly a tempo-synced tremolo plugin or other dedicated "pumper" plugin on the master.