Ice core data reveal that they max out at 300, and that's been the case for more than 600,000 years. I think the last time they exceeded that was around a million years ago, when they reached 800. No one is certain about the cause, but some say it might have involved major volcanic activity; still, it's estimated that it went up at a rate of 30 ppm every one thousand years.
The problem is that it's now above 400, and it's been going up at a rate of 1 ppm a year, or fourteen times faster. I think it went above 300 during the mid-1970s, which is why the 30-year global cooling cycle (which happened during the 1910s and 1940s) stopped.
Scientists don't know what's going to happen next, but they've detected over 50 positive feedback loops (phenomena driven by a slight increase in global surface temperature anomaly but also reinforcing the warming) the past two decades, and have no idea what might happen to things like the global conveyor belt.
Meanwhile, it was only realized around a decade ago (around 2013) that oil production per capita peaked back in 1979. Most of our food, manufactured goods, and services are heavily dependent on fossil fuels.
We live in very interesting times.