I'm an idiot. He has already got Posh promoted 4 times
Fergie's promotions were runners up in 2007/08, 2008/09 ( L2 to Championship) , 2010/11 playoff winners at Old Trafford (came back in Jan 2011) and 2020/21 as runners up
Since his first stint with Posh, Fergie has been relegated only once technically, as the other two relegations happened after he'd been replaced / left ( 2009/10 - replaced in December 2009, 2012/13 - relegated on GD with 54 points, 2021/22 - left in Feb 2022)