It is hard to find any hard sources for this online and I would very much like to see an answer from a person from the field (or someone with similar knowledge and expertise), but since there is, I suppose, quite a small chance that such person would visit this site and answer this question, I will try to answer nonetheless.
The biggest problem with information for calculating how many animals need to be killed for 1kg of cheese is the lack of figures on the yield of rennet from an animal's stomach. For this, I was able to find only a sort of anectodal evidence, based on the claims of one natural rennet manufacturer from Austria, which states that to turn 19,000 litres of milk into cheese, 2 calves stomachs would be required. There is also stated
1 litre of rennet extract contains approximately 1,000 mg of enzymes
and 18% of table salt. The rest is just water. For the processing of
10,000 litres of milk into 1 ton of cheese approximately 2 kg of
liquid rennet extract is needed. This means: 2 grams of natural
20% to 30% of theses enzymes remain in the cheese, the rest goes into
the whey and is completely destroyed by the drying process of the
There is only approximately 0,5 grams of enzymes remaining in 1 ton of
cheese. This means: 0,5 ppm (parts per million). 1 kg of cheese
contains therefore 0,0000005 g rennet enzyme.
On the other side: for the production of 1.900 kg of cheese (19.000
litres of milk) 1 cow and 2,7 calves have been killed: this means:
approx. 0,375 kg (dead cow) for 1 kg of cheese.
However, the 0,0000005g per 1kg figure is in contrast with much bigger (by three orders of magnitude) 0,0003g per 1kg which I have found at Wikipedia (although unsourced). I will leave for everyone to decide themselves whether this undermines the validity of the above statements or not.
All in all, the contents of animal rennet in cheese are probably not that big and could be written off as unconsequential not worth worrying about. But here should be mentioned that the rennet is probably the smallest factor for suffering and deaths of animals in the dairy industry (and cheese production).
While it takes the amount of rennet equal to two dead calves to produce 1,900kg of cheese, with milk production of about 7000 litres of milk per year, it also takes one full cow's life, which is spent by incessant milk production, intercepted by occasional stealing and murder of its offspring (may be for the rennet, but would probably be killed off anyway) and, on top of it all, artificially shortened to industry's average of 2.7 years.