Join Date: Jul 2001
Location: Near Pittsburgh, Pennsylvania, USA, Earth (PIT)
Programs: Airline/TSA Avoidance Platinum, Hotel Disloyalty Silver, Hertz 1.7*
Posts: 5,277
The highest one I have is $25k (MBNA Quantum Mastercard). Higher than that is certainly possible in theory but isn't really much more useful for anything I can think of. There are very few emergencies where you'd really need to be charging tens of thousands of dollars to a credit card all at once. Having multiple cards works just as well.
And yes, the limits are set by the banks. I'm pretty sure I've gotten an app or two in the mail over the years that suggested a possible limit of $150k, though I don't recall which bank(s) they were from.