We get some idea about this from the text when we re-read it:
Denth froze, then stumbled. Fifty Breaths fled from Vasher’s chest and
surged into Denth’s body. They would be unwelcome, but he couldn’t turn
them away. Fifty Breaths. Not many.
But enough. Enough to make Denth shake with pleasure. Enough to
make him lose control for just a second, falling to his knees.
“Nobody ever expects it,” Vasher whispered, stepping forward. “Breath
is worth a fortune. To put it into someone, then kill them, is to lose more
wealth than most men will ever know. They never expect it.” (Chapter 57)
We know that 50 breaths is what is required to reach the First Heightening, which would give Denth Aura Recognition. That in and of itself wouldn't do much, but 50 breaths is a sizeable amount.
Vivenna had at least 600 breaths given to her as she had Perfect Color Recognition (Third Heightening). We also know that she could sense how many breaths people had. Throughout the book, we see no evidence that Denth had more than just 1 at a time, let alone a large enough number for her to notice. He in fact seemed to prefer to have less as it made him less noticable to those with Aura Recognition (Chapter 35 in the cellar). How he could have a store and pull out only 1 at a time, I don't know, but he was one of the Five Scholars, so we don't know all that he knows.
Even though he had had large quantities of breath in the past, it wouldn't matter if he hadn't held a large amount in some time:
“It’s shocking, when you first get it,” Vasher said. “It’s usually not too bad
if you take the Breath back after only an hour or so. Wait weeks, or even a few
days, and it’s like taking it in for the first time.” (Chapter 43)
So, assuming that Denth avoided having large quantities of breath to avoid attention, we can conclude that he would get quite a shock from those 50 breaths. Imagine the amount of endorphins you have in your body, and imagine if someone injected 50 times that amount into you. I'd imagine you'd shake with pleasure for at least a moment, allowing someone to kill you if they wished to.