I've not particularly counted, but a fair few of my client horses get edgey when on too much sugar/starch or if the diet is just not right somewhere. Probably somewhere around 25% or more. I use the term 'edgey' somewhat loosely as the behaviours expressed can be anything from vague, distracted, incommunicado to quite aggressive kicking and biting. Inbetween are all the stereotypies including box walking, inability to stand still etc etc.
My own mare Grace got into deep trouble (prior to our partnership) probably largely due to her own sensitivities to sugar/starch. The pictures below show her after her previous carer spent four months trying to put weight on her in the traditional fashion. I acquired a horse that couldn't stand still when tied, couldn't concentrate, had trouble with canter and was extremely tricky to brush. (The list is endless, but this will give you an idea.)
Also note the terrible colour in the above photos.
Below - this is what she looks like when on her low sugar/starch diet with a decent supplement. And a year later, Grace can happily stand tied for ages, relaxed calm and happy. Check out the colour too!
Grace has no trouble cantering either! :-)
I often think that the horses which are really suffering remind me of people coming down from drugs. But they have a choice, our horses do not. And far too many horses get admonished for bad behaviour when the answer lies in our hands - in what we feed them.
Note it took me weeks to wean Grace off her addiction to all things sweet. There were tears (mine) and tantrums (hers). But I think you will agree it was worth it.
Now I have a horse that people comment openly on how well behaved and sweet she is - an angel no less.