Welcome to JOF! I don't think I've seen anyone post a definitive number for this but the depth of mattress is really only limited by the space. For the front bunk it can hold as much as you can stuff behind the straps; and you can always add longer straps. The back bunk is a little more limited because it has to fit behind the bathroom wall, but on our 19H there is probably an additional 2" there at least.
We have a 1" foam topper that was like $40 at Wally Mart that we use under the front mattress. I didn't even have to adjust the straps to fit that in. It was just enough extra foam to make it comfortable for us.
Remember that whatever you add, you have to be able to fold in half though.
2011 Jayco X19H (purchased 2015)
2008 Jayco 1007 PUP (purchased new, traded for the X19)
2018 Nissan Titan Midnight Ed.