For that many I'd seriously look into using electrolysis for cleaning.
If the rust isn't too bad you can do a vinegar bath too. 1 part water 1 part cheap white vinegar. Submerge entire pan and let set for 30-45 min. scrub with stainless scouring pad, repeat if required.
No need to get a massive bottle of Flax oil. A little bit goes a long ways. Also flax seed oil even when refrigerated goes bad too fast. The layers that you put on are super thin and as you get more layers on it takes even less to maintain. So buy the small bottle and use until it either goes bad or you use it up. We have 15-18 cast iron pans, dutch ovens lids etc, and I can't get through a whole bottle before it goes rancid.