I just got my 12/0 spooled with 100lb mono at bass pro. They had a line counter on the spool the whole time and I got 1080 yards on the reel. I had them pack it to the rails. However, This was a little too much line because when I used it I was not able to fit all of the line back on the reel at the end of the night. I would say this reel can hole just short of 1080 yds of 100lb mono. Hope this helps someone.
Thats Good Info Cause If You Google Up Penn Senator 12/0 Spec They Say You Can Only Get 875yds Of Mono. I Always Belieaved That You Can Get At Least 1,000yds Of Mono But With A Chrome Spool,... Does Your 12/0 Have One Or A Black Grafite Spool? 
Mine has the black spool. But it was put on tight with the machine.
Mines Is Also Put On With A Machine & Its Got I Belieave Ande Mono 100lb Yellow, I Found The Previous Owner & He Said He Caught 2 Hammers & 2 Tigers On It. Its Riged Up For Marlin & Tuna Fishing With 10ft Of Shock Cord Along With A 50ft Bimi Twist 
the line counter at bass pro is not always so accurate from my experiences
i know this an old post but i got my 5lb spool of 100lb clear bullbuster yesterday and put line on my 12/0 today. i already had 700yds of 100lb braid on the bottom from when i had it spooled before. so with the line machine i ended up with 900yds of 100lb mono on top of the 700yds of 100lb braid so i have 1600 yds total atm. im pretty happy with this so for the bhc i will be using that and my new squall 50vsw 2 speed with 1000yds of 50lb bullbuster on it. BTW the 12/0 is the chrome spool version.