I'm searching for the mammal species that can produce the most milk during lactation. I Googled it, but it says dairy cattle which biologically speaking is not right answer, because a baby whale can drink up to 150 gallons (> 550 litres) of milk per day (according to whalefacts.org). So which mammal can produce the most milk?
In animals, such as the blue whale, that fast during lactation, nutrients are mobilized from tissues (maternal reserves). Supply is mainly limited to body fat; thus, total available supply is proportional to body mass. Body weight of a blue whale is approximately 120,000 kg (at the start of the nursing period) and lactation duration in this species is 210 days (7 months). According a published paper (Oftedal 1993, J. Dairy. Sci. 76), "a female blue whale arriving at the feeding grounds weighing 80,000 kg might gain 40,000 to 50,000 kg before migrating to low latitude waters to give birth. This stored mass is assumed to be lost during the 7 mo of lactation; thus, milk production can be predicted to be 27,000 to 33,000 kg, which is equivalent to about 23 to 25% of initial maternal BW. During this period, suckling calves gain about 81 kg/d, or a total of 17,000 kg, which corresponds to a predicted milk intake of at least 26,000 kg. These two estimates are reasonably similar and indicate milk production of about 30,000 kg milk, or 140 kg/d". Production per cow is lower (in the United States it averaged 23,391 pounds for 2019, United States Department of Agriculture (USDA)).
About your second question (fat content), true seals (Phocidae) exhibit the shortest and most intense lactations, with extremely rapid weight gain in their pups. Hooded seal lactation duration is only 4 days (briefest lactation period in mammals). Hooded seal mothers secrete milk, which contains 60% fat, at rates of 10 kg/d (Iverson et al. 1995 J. Comp. Physiol. B 165).