Depends on what yo mean by Mega flow.
The number of drains, and pipe diameter can depend on the size pump you use, and how much flow you want to achieve (which also depends on type of fish you keep)
I you keep oxbow species like angels, discus, or Bichers, you won't need strong current,.
If you keep Tiger fish, Dorado, or Tomocichla, you may need quite a bit
I like rheophillic species like Tomocichla, Chuco, and Rheoheros, and for them, I need heavy flow., enough to have plants constantly bend in the current.

For the tank above, I used two, 1.5" drains.
For the 180gal below, with simple riverine species, I still use, two drains.
I have never found canister filters to provide a strong enough flow for the current I needed.