Given a particular block number, how can I calculate the Ethash used by this block?
For calculating the Ethash you need a lot more than just the block number (you need the entire header). The documentation about how this is calculated can be seen here.
You can copy paste the python code provided in the documentation in a file and pass the block header to the hashimoto_light function defined to calculate the Ethash. This needs no extra packages other than bare Python 3 so it should be easy to run and play with.
Hope this helps