Basic Rules, p. 57:
Short Rests
In general, over the course of a full adventuring day, the party will likely need to take two short rests, about one-third and two-thirds of the way through the day.
DM's Guide, p. 84
Short Rests
In general, over the course of a full adventuring day, the party will likely need to take two short rests, about one-third and two-thirds of the way through the day.
Sage Advice Compendium 2019
Is there a hard limit on how many short rests characters can take in a day, or is this purely up to the DM to decide?
The only hard limit on the number of short rests you can take is the number of hours in a day. In practice, you’re also limited by time pressures in the story and foes interrupting.
There is no argument that the design ratio is 2:1. Nor is there any argument that the majority of table have trouble getting to even that number, at least based on a perusal of enworld and other forums in general. As many tables have difficulties getting to the requisite 6-8 encounters a day, it is unsurprising that most short-rest classes fall short (ahem) of the number that they are balanced for.
Given the difficulties in creating DPR spreadsheets (and the amount of work that entails, given the necessary assumptions and work), it is unremarkable and unsurprising that no one has yet created one that fully takes into account resource management as well. That said, I look forward to seeing that at some point, as opposed to cherrypicked examples.
Until such time, perhaps someone wishes to start a thread discussing how overpowered all those shortrest classes are compared to the ridiculously underpowered classes like Paladins and Wizards, and see how it goes.