A lot

It started as multiplication in a maths class.

Then someone asked how many seconds there are in a year.

The students worked out how to do the sum then worked out the answer.

Somewhere along the line we got onto human cells and the fact they are being constantly replaced.

Somewhere around 20 million or so every second.

So, if you can say it, this is roughly how many human cells are replaced in a year (give or take the odd billion...)