What’s the advantages, if any, of playing only one whistle? Or do a mix of different whistles give a player more dimensions?
Advantage of one: you soon know it thoroughly, and have no adjustment interval when you pick it up.
Advantage of many: you can suit the volume, voice, and playing characteristics to the situation.
I like having a choice - I have a lot of cheapies, and a handful of midrange to high-end whistles. There’s times I pick up a Generation or Sweetone, and other times I reach for a Dixon, or Serpent, or Syn. I love my Dixon Bb for late night playing and for slow airs, but there’s time I want the assertive, furry, volume of a Serpent D, And there are some tunes (especially non-Irish pieces with a lot of tonguing in the second octave) where the Serpent doesn’t work as well as the Syn. Though I love both the Oak and Walton’s C whistles they sound very different (as do the Generation and Dixon Bb). And a whole lot of other tradeoffs can be made.
And though some whistles get more play than others, almost every whistle I own gets played regularly.
What’s especially nice for a former Sax and Guitar player is that the whole lot cost me less than $500. So why not have a selection?
I tried the one whistle method before, and I actually played LESS because there wasn’t the choice of different whistle for different times. I also like having at least a few to choose from.
However, if you find yourself wanting to give away all your whistles, I am open to donations. I’m sure TelegramSam wouldn’t mind, either. 
When you play one whistle you get to know exactly what you need to do to get a particular sound, and it gets to be automatic. This is a definite plus.
But that doesn’t mean the sound is the one you want, and you may need to go to another instrument to get the variety you want. ‘This whistle really cuts through and can be heard, but at home I need to be quieter.’ Another whistle might fit your breathing to get a high B like a glove fits a hand, but can’t be heard if there is an accordion near you.
I waver between the two ideas.
Also, I find that if I play a different key in practice, specifically if I practice on a C whistle, when I pick up the D for a lesson I have more lung capacity than if I practiced solely on a D. I’m not sure if this is ‘cheating’ or not.
Anyway, when I started taking lessons I was told to pick one and stick with it, and for the most part I do that. Hence the significant drop in the whistle stock market since last December.
When dealing with non whistle playing serfs
Advantages of One Whistle: no mocking from non-whistle playing circles
Advantages of Many Whistles: The look on their faces when you pull em all out
When dealing with other board members:
Advantages of One Whistle: You know it really well
Advantages of Many Whistles: the strange feeling of power 
I’m just on a useless comment spree right now, please forgive me 
disadvantage by playing one whistle:
tragicomedy if you don’t know what you really want on whistle.
advantage:
secret…
p.s.Congrats jonharl, about your new Sindts.
playing one whistle is as bad as playing too many. but only play your favourites.
with only one whistle, your body position, like all the rest stays the same.
if you run out of wind on a low D, you can switch to low G, then high D. that way you can practice more,
and the various sound with rapture, they fire you!!
Lots of whistles is more fun. I’ve got high end, low end, and middle of the road whistles and I enjoy nearly all of them. Depending on my mood at the moment, I may simply want the sound of a traditional Clarke. Sometimes, I just gotta play my Sindt. I’m lucky in that I also have a stream of wooden whistles passing my way and although they are made to the same specs, each of them is a tad different too, which is interesting.
we all know that “the one whistle” is made of mithril. I asked Colin to make me one, he’s still searching for an alternative source… for he dares not enter the mines of moria…wuss.
Disadvantage to playing one whistle . . . not knowing the antisipation of a package being at the post office.
Advantage to playing one whistle . . . more money in your checking account. 
If, like me, you play in a lot of different contexts, you have to have whistles in most, if not all keys. You can minimise the adjustment required by choosing a single make known for it’s consistency. For me, that was Overton. You’ve got to like the sound though, of course.
One thing I do with whistles, in non-Irish contexts, is layer several whistles in the studio. I love the way a Copeland or Overton blends with the purity of a Burke or Sindt.
Playing a tune on a low whistle really helps me to play it more easily on a high whistle. I like that.
On some days I just seem to have trouble with a particular whistle. I don’t know why but it just isn’t happening for me. Having a choice of two or three in that key means that I can just pick up another and off I go. I have absolutely no idea why this happens.
In most keys I play in a lot I have two or three whistles, chosen to give me that choice of sound when layering or just playing alone. In some keys though it’s getting ridiculous, high D especially. But when I think of which ones I’d part with, I just don’t want to part with any. Even a whistle I didn’t like and didn’t play for a few years, I pick up and suddenly I like it.