Ask the folks at Department of ... whichever the department that already has this information. There is no need to start from scratch if this information is already available from a trustworthy source.

But, if this department doesn't exist, then we can start by:

- estimating the population of Canada

- the number who drink cow milk and how much

- the percentage of milk imported

- the percentage of milk exported

- the percentage of milk wasted

- the percentage of milk used in other products

- the amount of milk produced by a cow

- from these numbers, get an estimate of the number of cows

It depends.........