Well your half right.
Flats are actually not 'considered' at least by DSS when it comes to noise and hot pixel reduction. That's what the darks ane bias are for.
Flats are considered for mitigation of vigeneting, and dust motes, spots, etc.
I typically match the number of flats to the number of light frames. So if I have 15 light frames I use around 15-20 flats, and other calibration frames. I keep librarys of 30 or so for each color channel so I will usually have enough. Darks and bias are even more extensive with different durations, temps, binning etc. I shoot the dark calibration files with the camera off the scope inside a darkened cool room on cloudy nights
As long as nothing has really changed in the arrangement of the optical train and its orientation you CAN use the same flats over and over regardless of target, duration, etc.
No need to shoot flats every night you shoot.
I have been successfully using librarys for darks, flats, and bias files. The library gets updated about 3 or 4 times a year unless something changes.
The results speak for themselves