Just image how many buckets of milk it takes to fill this car.
We saw this at the Railroad Museum in York, England.

Found this fun fact on Reddit.
How much milk can that tank hold?

How much milk does the average cow produce?

So first to the tank size:

Guesstimate of size is that the tank is a cylinder 2m in diameter and 4m long, giving a total volume of 12.57m3.
This is equivalent to 12,570 litres (or 2765 imperial gallons).

Next on to the cows:

Taking data from here, it looks like a cow produced 7,844 litres of milk per year in 2014/15 (latest year with hard figures).
Various other sources suggest cows are milked 2 or 3 times a day.
Assuming cows produce milk at the same rate every day, the above suggests cows produce about 7 litres per "transaction" (assuming 3 milkings per day).

Final answer:

This means you would need a herd of 1756 cows to fill this up in one go, or 586 cows to fill it up in a single day.