How many gallons of DEF does a typical tank hold?

most road trucks are 16 to 21 gallons. Depending on manufacturer and model.
It may not be low. It may be the QLS is going bad. Could also be a plugged filter, weak pump or the dosing injector any number of things. It should set a code for the problem. Will need to connect to it with a computer to read them.

 

They did 3 regens at our shop on my truck,hooked up to the computer.It tested everything in the system and the codes cleared and didn't come back.
Forced regen.

 

The most I ever did was about 10

 

I think my last truck had a 12 gallon DEF tank. I don't recall. I routinely only filled it every 3rd fueling. I routinely only toped-off the DEF when it got down to 1/4 full.

 

Mine holds about 30 gallons..

Your problem could simply be a plugged vent. All you have to do is remove the cap after running the truck awhile..if you hear a vacuum sound when taking off the cap, your vent is clogged.

 

Mines a 2019 Freightliner, DD 13.
It has to be connected to a computer to do it.It took about 3 hours to run it 3 times before everything cleared up.
It gets very hot when they do it.

 

It cleared all the codes and runs great now.

 

My 2022 Cascadia holds 20 gallons.
But our DEF systems all need trashed! There's just as many downsides as upsides (probably more down) I'd take that entire system off in a heartbeat if I could.