bo: that depends on their expenditure and obligations
That depends where they live and what they do
I would say that £20,000 is but it depends on some factors
There are people who make six figures who aren't financially stable. Income is one piece of financial stability. You also have to take into account spending. Do you have student loans? A car loan? What about your rent/mortgage? What are your spending habits like? Are you disciplined about saving money? Or are you living above your means?
Being financially stable means that you can handle life's financial ups and downs. In order to do that, you should have an emergency fund set aside (enough to live on for 2-3 months in case you lose your job). You should have a reasonably secure job that provides an income you can live on comfortably and you should be able to pay all of your bills on time, without issue. You should be able to put some of that money away into a retirement fund (say, 10%), so you don't have to work every day until you die. That's a pretty decent baseline for financial stability.
Their income is irrelevant, its their net income after expenses that matters. As an example, professional athletes and musicians are notorious for earning large amounts of money and going broke. You could be financially stable down by the poverty line if you have low enough expenses, rare but possible.
It's whatever is enough to pay their bills, with some savings. That number will be different for different people.
as much as it takes to pay their obligations each month and maybe have a little left to save
There's no single global stable income – it depends on the location and lifestyle
Sufficient to meet current needs, including savings and retirement.
To be financially stable simply means your receivable income is equal to or higher than bills payable. For a frugal person in a small town, that might be as little as $25,000 a year.
It would be enough in their area to afford housing, to provide enough food to not go hungry, to have a reliable vehicle, and to afford the occasional luxury.
I can't give an exact number since that is going to vary from place to place. A person making 40k a year would do quite well in say Salisbury, while that same person and income would have a much harder time if they lived in NYC.