What is the theoretical limit a day trader can make from the current stock market in one day?

The answer ranges so much from -$100m to +$100m. It depends on your skill and account size. There are extreme cases of more on both sides.

As a side note, people tend to dream about the lambos and not think about losing all their money (or being homeless in extreme cases). Without risk management, the latter is very possible, even very likely!

I would say the probability of losing all your money is more likely than the lambo. This is a very hard business, 95%+ failure, people assume they will be in the 5% that are successful, when statistically speaking, not all can be in the 5%. If you can survive 3–5 years and still have a trading account AND money to pay your bills, you have a chance.

It’s really more mental than anything, like a professional athlete, it’s not something you dabble in, it requires full emotional and mental attention, otherwise you have little to no chance of success.