Debate ratings: 48 million, which sounds like a lot, but that's down from the 73.1 million who watched the first debate in 2020
WaPo:
On Thursday, 8.8 million people watched the debate on Fox News, 8.7 million people watched on CNN, and just under 4 million people watched on MSNBC. Had CNN not allowed other networks to air the debate, CNN probably would have attracted a larger audience.
The total television audience of 47.9 million people will probably increase when the Nielsen ratings agency releases the final results later Friday.