The capacity of fixers vary. The data-sheet should give you a rough indication of how many square inches/cm/whatever of film or paper per liter or gal. a particular fixer is good for.
But, you still need to test because there are many variables, T-grain films vs conventional, for example.
FWIW, the most economical fixer I've found is C-41 fix from Kodak, it comes in 5 gallon bags of concentrate. Expensive to ship, but cheaper than shipping 5 one gallon containers 5 times (at least it used to be). Also, since Kodak doesn't publish info for use with B&W materials, you need to do your own testing for times and dilutions using a retained silver test. The other downside is that the packaging is intended for mini-labs, storing 5 gallons of concentrate is a bit of a pain, and the required pouring spigot doesn't come with it.
If you aren't interested in "test pilot mode", TF-4 and TF-5 are reliable, also Ilford Hypam is good, and reasonably cost-effective, another good one to consider would be Sprint.