The heaviest Ascent is 4,602 pounds. The maximum GVWR is 6,000 pounds (all trims). Maximum tongue weight is 500 pounds.
Subaru has released a number of videos of them testing it towing a 4,300 pound Airstream and passengers. That's a 405-410 pound tongue weight on the Touring trim they used, leaving just shy of 1,000 pounds for cargo.
Ideally, a trailer should be 4,000-4,300 pounds or less (80-85% roughly) for the Premium trim levels upwards. The reviews from the people who tested the towing configuration I mentioned above are pretty good.
Here's one of a few videos showing it towing 4,300 pounds pounds, plus a couple passengers:
I'm shooting for 4,000 pounds max (trailer GVWR), maybe a little more.
You'd need to factor in trim levels, cargo, passengers, etc. For instance, the Premium can tow the full 5,000 pounds with more cargo weight available in the Ascent itself because it's 125 pounds lighter than the Touring model.
As an example of the math, with a Touring:
+6,000 pound GVWR
-4,602 pounds "empty" weight
-500 pound tongue weight
--------------------------
898 pounds left for car cargo and passengers
As an example of the math, with a Premium:
+6,000 pound GVWR
-4,477 pounds "empty" weight
-500 pound tongue weight
--------------------------
1,023 pounds left for car cargo and passengers
If you needed more cargo weight, you'd need a lighter trailer than 5000 pounds/500 pound tongue weight.
Remember, that's the loaded trailer (GVWR) weight - not the empty or curb weight.
AND REGARDLESS:
If you're doing regular or long distance towing, I for one never like running at the absolute maximum rating. I'm shooting for 4,000 pounds, because it's 80%. Many people recommend staying at or under 80% or 85% max for a trailer that will be towed long distance or somewhat regularly (a couple weekends a month throughout the year, for instance).